Can you imagine being the first guy to be convicted because of fingerprint evidence? (or more modernly - DNA evidence)
“You got nothing on me coppers! Wait. Um, what’s that you say? Everything I touch?!?! Awww crap.”
Now the state of Florida is taking DNA samples from every single Felony arrest just cuz. And yes, that is “arrest” not “conviction”
Now fingerprinting is not considered a 4th amendment violation because it “involves none of the probing into an individual's private life and thoughts that marks an interrogation or search.” (Davis v. Mississippi, 394 U.S. 721, 727 (1969)) but doesn't DNA sampling go a little further than that? It requires taking a piece of you, not the equivalent of an image of you (like a fingerprint). There isn’t anything more private than your self. If there is a good reason to get the sample then go ahead. But you cant just go get one from everyone just because you can and its easy.