If you accept the estimate of the Supreme Court in City of Los Angeles v. Patel, there are about 2,000 hotels in America’s second-largest city. That’s a big number even for a big city, but the category presumably runs from the Four Seasons through your basic Ramada Inn and ultimately down to the less savory places where rooms rent by the hour for quick trysts (sincerely romantic or honestly commercial) or perhaps longer if the guests want to trade or package or smoke their dope at leisure. But for over a century all such hotels have been subject to a city ordinance requiring the host to maintain a registry (paper or digital) with lots of information about guests—name and address, car information, date of arrival and departure, room assignment, price, method of payment and often credit card and driver’s license data. More controversially, the ordinance requires the hotel to make this registry available at all times for unannounced ad hoc inspections by the police. And some hotels believed that the ordinance violated their rights under the Fourth Amendment.

You might have expected the case testing this law to arise when the police learned about a particular guest doing bad things in Room 222, and the guest, charged with a crime, then moved to suppress any evidence found there. Or perhaps the hotel manager or desk clerk might have been implicated in the crime and was the one trying to do the suppressing. But this case had no facts at all. In legal jargon, it was a “facial challenge,” a civil case brought by a group of hotels in a preemptive strike against the city, with the goal of an injunction against ever applying the law.

Of course the case has some doctrinal niceties that only a lawyer could love: One was about the very idea of a facial challenge—the question whether there were any conceivable legal ways of applying this ordinance, in which case a facial challenge would have failed at the start and the issue would have awaited for an individual complaint of privacy abuse. But in this week’s decision the Court found so little constitutional basis for the inspection power that it flat-out struck down the law for all cases—and all manner of hotels. So let’s turn to see what the Fourth Amendment means here.

Contrary to popular perception, the police do not always need a warrant based on probable cause to enter a building and do a search. A law school course in the Fourth Amendment probably spends more time on the exceptions to the warrant requirement or the probable cause requirement than to the requirements themselves. But one big exception is for the vague area called “special needs” cases and the subset called “administrative searches”—i.e., searches of certain business premises, not homes. Under this doctrine the Court has sometimes relaxed the warrant or probable cause rules and allowed fairly ad hoc searches where the relevant business is part of a ”pervasively regulated industry.” So far, the examples of that category are liquor sellers, firearms dealers, mines, and auto junkyards. The rationale is that the need to protect public health or safety through regulation of these types of business is especially acute (and, the Court sometimes says, the chief aim of the law is non-criminal and the criminal law consequences only a byproduct). A related rationale, which you should feel free to view as tautological, is that the industry is already subject to so many other forms of regulation that it is disingenuous of the business to now argue that there is much privacy left to protect anyway.

In Patel, the Court breezily declared that the hospitality industry bears no resemblance to these others. Justice Scalia didn’t see much hospitality: he looked to the less savory end of the hotel spectrum and summoned images of prostitution, human trafficking, and immigrant smuggling. The majority thought the inspection requirement a serious intrusion on the autonomy of the hotels and that law enforcement could work very well even without the power to inspect at will. The city—and Justice Scalia—took the view that the intrusion on the hotels was slight and the need of law enforcement for these inspections was strong. (Justice, Scalia, as is his wont, also delivered a history lesson about how even in colonial times ye olde inn was subject to all kinds of nitpicky regulations. The majority responded that ye olde inn of the time did not collect digital credit card data.)

The heart of the matter is that, the theoretical warrant requirement aside, Justice Scalia believes that the Fourth Amendment is all about reasonableness, and that reasonableness is a matter of context and practical understanding of social circumstances. And, on balance, he thinks this law is, well, reasonable. And the majority says it isn’t reasonable at all, at least the way reasonableness has been framed by the administrative search doctrine.

So what must the city do now? Well, if the police really have probable cause to believe that drugs or pimping—or maybe even insider trading—is occurring in the local hostelry, they could get a regular search warrant, or, perhaps make out a traditional case of exigency for skipping the warrant if the volatility of the circumstances is extreme.

But obviously the ordinance was aimed more broadly at enabling the police to deter crime by letting the hotel (and presumably guests) know that they could check the registry any time, for no special justification, and also enabling the police to more efficiently investigate crimes they are already alerted to. And now the Court has taken this tool away. But the Court says a compromise remains possible. The city just has to allow for some possibility of “precompliance review” whenever a hotel objects to inspection. That may mean something more like a subpoena than a search—i.e., the hotel must be given reasonable notice that the police want to look at the registry and must have a chance to explain to some judge or magistrate that there’s no good reason for the inspection. (Alas, the Court doesn’t say just how much explanation the police have to give, except for the vague hint that the hotel can argue that the motive for the request to inspect is “illicit.”)

The city objected to even this modest compromise, arguing that the review process would allow someone with an interest in doctoring or erasing the information to do so. The Court responded that the police could “seize” the registry without reading it, while the permission for the reading was sought. It’s typical of the case that the justices have radically different views of the particularities of this situation. The majority thinks it’s a reasonable way for the police to keep the status quo for a short while, but Justice Scalia suggests that having the cops standing guard at the reception desk and blockading the registry would be lousy for business and scary for the guests.

So on this point, as with the broader question of whether this counts as an “administrative search,” the debate is a matter of empirical hunches, impressionistic assertions about social behavior, and back-of-the-envelope weighing of costs and benefits. Welcome to Fourth Amendment jurisprudence.

The vote was 5-4, and while Justice Sotomayor wrote the majority, inevitably it was Justice Kennedy who decided that the lovely places we call hotels in California should be more welcoming to guests and less so to the police.

A final note: I have described this case pretty neutrally here, but on one issue neutrality must yield to incontestable principle. Late in her opinion Justice Sotomayor says that the ordinance must be struck down in part “because it fails sufficiently to constrain police officers’ discretion as to which hotels to search and under what circumstances.” To this statement I respectfully dissent—actually I do so disrespectfully. The substance of the point is fine. But to say “it fails sufficiently to constrain” is to commit an unforgivable crime to the language by invoking the mythical split-infinitive rule. (Should we credit the ordinance for at least failing sufficiently, as opposed to insufficiently?) Even the iconic Strunk and White dismiss the so-called rule that produces such syntactical barbarisms. Putting the adverb right after the “to” avoids a gratuitous ambiguity and sounds far better. The split infinitive rule seems to be especially beloved by lawyers. Justice Sotomayor seems to think it sounds fancy and professional to keep the adverb before the infinitive. I say it sounds ridiculous.