The issue I have with all these complaints is it seems like the rule is this: According to Slashdot's readers, the police are allowed to do something if it's hard and expensive, but they're not allowed to do something if it's easy.
For example, the police are allowed to spend thousands upon thousands of dollars secretly tailing Tony Soprano, seeing where he goes and who he meets with. However, they're NOT allowed to put a GPS on Tony's car to do exactly the same thing.
When law enforcement becomes too easy to do, it causes problems, because EVERYTHING is illegal in some way or another, and the only thing stopping the police from arresting everyone is that they have a limit to the number of man-hours they can invest, and they triage in a way that (at least for now...) most people consider reasonable.
For example, you can (probably) meet up with your local pot dealer without having to worry that the police have spent thousands of dollars putting a secret tail on him, because he's small potatoes and you're even smaller. However, if they could generate a free interaction network computationally by following the GPS coordinates of every person in the city thanks to their cellular telephones, it would only be a matter of time before someone put increased emphasis on enforcing the "war on drugs" and one enterprising cop did five minutes of datamining and got a bajillion arrests to add to his record.
If nothing anyone ever did was illegal, sure whatever, the police can arrest all the criminals in the world.
Unfortunately, the laws aren't like that. You probably did a few illegal things driving to work this morning.