The American constitution exists to say that there are some things the government ought not do, even if it makes their job harder, and that if you truly must do those things, you have to convince more than 51% of the country that it's okay. Californian cities can take their own people's guns away, but it is not their place to take them away from rural Kansas, where a cop is an hour away at best. You have to have at least minimal evidence that somebody did something before you can ransack their house.
I readily grant that our current practice is to play silly games to say that these lines aren't being crossed when they are (border phone searches, stop and frisk, you name it).