This makes sense in principle, but in practice police cannot both carefully adhere to the law and serve their intended function.

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.