situation
The police are trying to solve a rape crime and have a suspect, but not enough evidence to get a warrant.
solution
arrest the suspect for failing to properly stop at an intersection (yes they can actually arrest you and book you for that instead of just issuing you a citation) and get their DNA sample. Now the government has your DNA on file forever even if you were innocent.
Yeah, if you never violate the law you have noting to worry about; however, that is not how our judicial system is supposed to be. The whole "if you are innocent you have nothing to fear" takes a Constitutional nose dive when taken to its logical extreme of "why not just DNA sample every newborn?"
Like I said, I don't have an objection to this specific ruling, but it could become troublesome if it is stretched any further(as SCOTUS rulings often are).