(1) Although DNA should be protected in the sense the government cannot take it from you, without a warrant (or claims your a terrorist). It's difficult to argue that if some hair falls off my head in public, should it not be considered "fair game"? Picking up papers out of the trash has always been fair game for law enforcement, so why should the hair be different?
(2) Unreasonable search and seizure is relative, it's not reasonable to pluck a hair off someones head. The same way you cannot go through someones office without reason. However, if someone disregards the hair, then it does seem reasonable to take the hair, because it is not invading a persons time/space/property/anything.
(3) It is a powerful tool to fight illegal activity. I have trouble believing a judge doesn't/wont (to some point) empathize with this and have a slight bias.
I find mass DNA collection very scary, but I don't know if this is fully valid or winnable case.