Police are expected to be the security force of the powerful and influential against society's undesirables. If cops were required to strictly obey the law, they would be much less effective at this job. Most officers would understandably opt simply not to get involved in situations which might lead to someone's rights being violated.
If the powerful want a force which is willing to knock heads for them, they need to assure that force it will not be held responsible for the mess it makes along the way.
You're describing the principle of qualified immunity, which is already in effect in US courts. I was suggesting an explanation for why this is the law of the land, despite it being in such obvious contrast with the principles of a free and democratic society.
If you don't agree with my explanation, I'd be interested in yours: why do you think cops are exempted from having to follow the law the same as other civilians?