The problem isn't that they're collecting DNA, it's that they're linking it to an individual for future identification.
Sure, I left my DNA all over some place I visited, but nobody has any idea who "I" am based on it, in the same way they don't have my fingerprints on file.
EDIT: Also note the DNA of everyone born after 1983 in California is on file.
As they are taking photos of the same people so that there is not question as to their identity in the future. What makes DNA different than the photo?
I don't know about you, but I can change my appearance a lot easier than I can change my DNA. Even this aside, a DNA link to your identity is a lot more accurate than just a photo.
Sure, I left my DNA all over some place I visited, but nobody has any idea who "I" am based on it, in the same way they don't have my fingerprints on file.
EDIT: Also note the DNA of everyone born after 1983 in California is on file.