Someone's rights(within reason to what they did) are forfeited when they are CONVICTED, there is a HUGE difference there. Otherwise if a murder happened in a small town the police should be able to collect evidence from everyone in the town, after all it could have been anyone, and that person who did it doesn't have rights at all, right? And if someone is proven innocent later is their DNA purged from the database or does it stay there forever just because someone suspected they committed a crime?
In fact, I don't think it's anyone's right not to be identified by the epithelial they leave lying around.
And I believe they do have the right; now how are you going to tell me I'm wrong? There is no legitimate gain from collecting DNA from a person if they where law abiding, or did not commit a crime where DNA was involved at all.
In fact, I'd encourage everyone to get registered for fingerprints and DNA, because the time, money, and grief it will save your family when your unrecognizable torso is dug up in the woods, is significant.
That seriously the best answer you have? If you really did leave DNA around wherever you go why couldn't they compare the body to samples taken from another piece of clothing or something you owned? Hell, if it was a relative you could just run a DNA test with a sample from the relative and when it comes back with a 99% odds they're related be all set. Your argument does not stand on it's own.
I swear to God, if you believe this is a good idea I hope there's a special place in hell for you. This is called unreasonable search and seizure, police even when executing a warrant must only collect evidence pertaining to the crime. This law pertains only to evidence collection that does not pertain to a crime otherwise it would have been collected in the original investigation. My body, my fucking epithelium! If I did commit a crime and DNA was valid evidence in it go ahead and keep it, you'd need to in case of appeal anyways, but otherwise you have no right or grounds to keep it, so do the right thing and destroy it.
An authority is a person who can tell you more about something than you really care to know.