I'm not sure if the slope is as slippery as the ACLU presents it as being. The government has always been able to do things under the guise of intelligence/prevention that it has been unable to do in the name of law enforcement (many would argue that the purpose of a stop/search is what makes something 'unreasonable,' and thus requiring a warrant, under the 4th amendment—searches for the purpose of law enforcement being presumptively unreasonable, while searches for prevention being more reasonable). The FBI, in fact, had a rather firm firewall between its law enforcement and intelligence division before 9/11, with virtually no sharing of information between the two. After 9/11 this firewall was highly criticized, on the theory that better information sharing could have prevented 9/11, though this logic would seem to argue in favour of information flows from law enforcement to intelligence, and not necessarily vice versa.
Right now all of these surveillance programs are being used for intelligence purposes alone, and I think it would take a very deliberate, very considered decision to apply them to law-enforcement contexts. Certainly I can't imagine it happening on the federal side anytime soon, and outside of the federal government I think only NYC has anything approaching this sort of infrastructure at their disposal.