Because whoever has the largest force wouldn't agree to it. For instance, if these counties chose to ignore all the rules and join Idaho, the U.S. has a very la

Because whoever has the largest force wouldn't agree to it. For instance, if these counties chose to ignore all the rules and join Idaho, the U.S. has a very large force that it can use to prevent this from happening. In fact, it doesn't even have to use the force, simply having the force is probably enough.