I think the decision was the right call. A man arrested for felony assault by pointing a shotgun into a crowd is later charged with rape after his DNA matches a sample from a cold case. How was his privacy violated? He argued that if authorities are allowed to take DNA samples from people arrested for major offenses with probable cause, that it violates privacy and will lead to an abuse of that power. I don’t think a person in that situation can claim that broad of a definition of privacy; police do have to fight crime and protect society, as citizens also have the right to expect to be relatively free from crime as much as they have the reasonable expectation of privacy. As for abuse of power, I don’t think you can say it’s absolutely guaranteed to happen in this situation. And if it does, you can argue just that on your own suit; you have recourse, you can make your case and argue the authorities overstepped their bounds. The point is: assuming that negative unintended consequences will always result, that slippery slopes, rather than being exceptionally rare are always unavoidable is to live in fear and to underestimate the abilities and desires of society and US government. Society loves stability, and here in the US we are rather stable due to our police, courts, laws, and government. No, it is not perfect and we should always be vigilant to abuses of power, but we can’t live in fear of (or legislate for) hypotheticals.

It’s an example of a continuum fallacy, where you (essentially) reject a rule as valid because of its exception(s). Also: If a=b, b=c, then a=c. But that doesn’t mean a=d or that a will eventually =z.

In the physical world as well as in social science, chain reactions are rare; things almost never discretely, spontaneously transition from one state or category to the next, let alone to the next and the next and the next etc. An extraordinary amount of continuous input and effort is required for that to occur in most situations.

And of course the irony is that DNA evidence has been effectively used to exonerate people falsely accused and incarcerated.