I don't see it as different than a cop. It is doing exactly what an officer can do, from the position of a government vehicle, and therefor should be treated as an extension of the agency or as an officer. If its legal for an officer to run every plate he sees then let the computer do it.
Otherwise stop it, a computer is no different a tool than a dog.
I have been thinking about this lately. I see your point and it's hard to argue with, but I think there is a line somewhere where the nature of the surveillance changes because of the scale. We've got a lot of legislation and jurisprudence that is probably implicitly based on the assumption that not every corner has a cop sitting there watching everyone 24/7, and that constrains the potential for abuse a bit. Crank it up to virtual, unsleeping cops everywhere, always, collecting and storing all information without regard to active cases, and it gets out of hand.
Maybe we should re-examine the analogy -- are computers with perfect vision, that never look away, and perfectly record any observation they are able to make, really comparable to cops? Are the resulting huge bodies of data that can be retroactively searched, really comparable to anything that existed before?
This is a half-developed thought admittedly; I'm not sure it's a good argument, or what should be done.
Otherwise stop it, a computer is no different a tool than a dog.