zlacker

I would suspect it's a Hobbesian thing - we allow government to do things closer to a stage of nature (e.g. locking people up) and deny that pleasure to ourselves. As such because government employees by their nature do things with our consent that we can never do the potential for nuisance suits arises.

So if I grab someone off the street and handcuff them we can be almost 100% sure I'm committing a crime. If the police do the same thing we can be almost 100% sure they are not committing a crime.

Exceptions happen in both cases but it's not unreasonable (which isn't to say it's correct) to take make laws that take that into account.