> Is it legal for US citizens to all put cameras on their cars and set up a network where we can all see where all police are at any given time?

Sure. There's absolutely nothing illegal about monitoring the location and activity of police (unless you broke some other law to do so, like hacking).

Hell, most of them will simply tell you where they are and what they're doing, as they tend to broadcast over unencrypted radio (which is also not illegal to listen to).