I'm not sure what's so hard to understand about this. Say things were done your way.
Imagine you're a crack dealer. The police ask to search the back of your car, you tell them to fuck off and come back with a warrant. They say they will.
Now you know that they will come back with a warrant to search the back of your car. Obviously you're not just going to leave your crack supply in there, you're going to hide it somewhere else. Obviously they're never going to catch you at this rate, they're never going to prove that you're in possession of drugs. But that doesn't magically make you an innocent, law-abiding citizen, you're still a criminal who belongs behind bars.
So if you're going to restrict law enforcement in such a way where they can never investigate anything without announcing it in some way ahead of time first, why fund them in the first place? It's not like they're going to catch anyone, any retard can just outmaneuver them easily then.