If you are interviewed at a police station, how is that not invading a persons time/space/property/anything if the intent is to steal some dna left on the chair? It sounds to me as using subterfuge in order to take someones DNA, and subterfuge should not allow the police to circumvent the law. That way makes the law meaningless, and that surely can't be the intent of the court.

Either give them direct permission to take DNA without cause, or make it illegal regardless of what loop whole they can think up.