But a cop who demands your DNA without a warrant isn't abiding by the law--he's violating the law of the land, the Constitution.
I'm glad you did say "a cop." If you don't report the bad cop then you are doing us all a disfavor. I've never had to report a bad cop because I never personally encountered one.
There are certainly bad cops in this country. Training cops is the job of the local academy. They are only as good as their city/state government makes them be.