Taking DNA samples upon arrest

There is a somewhat controversial proposal in Washington State to take a DNA sample from people when they are arrested, regardless of whether or not they are charged with a crime (much less convicted).

I confess that I don't understand why this is controversial. If it's OK to take fingerprints from arrested people (and it seems to be), then a DNA sample is really nothing more than a far more accurate (and more commonly left-behind at the scene of a crime) means of identification. As long as the DNA sample is, like fingerprints, used strictly for identification purposes (rather than, say, determining hereditary diseases or other genetic information, which absolutely would be a violation of privacy), I can't see how this would be legally distinguishable from fingerprinting.

After all, the taking of fingerprints upon arrest is to enable identification, so the legal underpinning for this practice must supports the notion of enabling future identification. If that's the case, then DNA samples meet the same bar in terms of what it accomplishes.

Conversely, if DNA samples are somehow unconstitutional as an identification mechanism (again, assuming that it isn't used for anything beyond that), then it seems to me that fingerprinting must also be unconstitutional.

I usually agree with the ACLU's point of view on things, and I'm certainly not wild about increasing the government's ability to keep tabs on citizens, but I don't understand the distinction the ACLU proposes between fingerprints and DNA samples in opposing this measure.