Last week the Supreme Court decided, based on the majority's appeal to reasonableness, that police can collect a DNA sample from people when they are placed under arrest. Scalia and three liberals formed the dissent, a strange set of bedfellows if ever there was one. Your libertarian-leaning friends are no doubt up in arms about "DNA databases" and the police state. To a great extent, hyperbolic language like that obscures the real problem with this decision.

In the majority opinion, Anthony Kennedy says:

DNA identification of arrestees is a reasonable search that can be considered part of a routine booking procedure. Taking and analyzing a cheek swab of the arrestee's DNA is, like fingerprinting and photographing, a legitimate police booking procedure that is reasonable under the Fourth Amendment.

Here is why that analogy is inapplicable and intellectually dishonest.

Fingerprinting and photographing are done for the purpose of identification. They're standard police procedure to ensure that your are Joe Blow rather than John Doe. They are not used retroactively to charge you with unsolved crimes. Contrary to what Hollywood portrays, useful fingerprint evidence is rare in criminal cases; the police don't get your picture and prints and then say, "Let's see what else we can charge this guy with." With a DNA sample, they will indeed have the ability to compare to a database of DNA evidence collected in previous cases – solved or unsolved. If you bleed all over the door handle during a home invasion in 2013 and get arrested five years later for drunk driving, the DNA evidence you provide at the time of arrest could be used with relative ease to tie you to the earlier home invasion.

So what?, most people say. Collecting DNA evidence will lead to convictions of guilty people in unsolved crimes. Good. But here's the real problem: this decision is, in essence, the end of the need for a search warrant. Let me explain.

You're suspected of a homicide. They neither have enough evidence to arrest you nor enough probable cause to get a judge to sign off on a search warrant. The police want to search your home to collect both physical and DNA evidence against you. But they can't.

Except now they don't have to. What they do instead, per this decision, is continue to keep you under surveillance as a suspect. They follow you around and wait for you to commit the most minor offense that will enable them to place you under arrest, even if they have no hope of convicting you. They collect DNA samples at the time of arrest and use that to connect you to the crime to which they couldn't otherwise connect you.

You're still not concerned, right? Because once again we're tying guilty people to crimes they committed. The problem is, law enforcement collectively has a strong incentive to collect as many individual DNA samples as possible. It is in their best interest to arrest, fingerprint, and DNA sample as many people as it can, both to resolve old unsolved cases and to provide them with a larger pool of suspects for future cases. I'm not saying that the FBI and police will be playing a game of "Find the arrestable offense" from now on. If they're smart, though, that's exactly what they'll do.

The problem here – ethically, not legally – is not that DNA evidence can be used to connect offenders to all of their past crimes. The problem is that in order for law enforcement to maximize the utility of DNA as a crime-solving tool, they need (theoretically) a sample from everyone. And there's one great way to get that, in the words of the majority opinion.

tl;dr = It is in the interest of the police to arrest you for something minor now to collect evidence that may tie you to a more serious crime in the future, as well as unsolved crimes from the past. Goodbye search warrants. You had a good 230+ year run.