are enforceable, even in right to work states (most states are, btw). Only way to beat them is if they are overlybroad in terms of time, or geographical area, etc. Certainly, an SEC school wanting to limit their coach from going to another SEC school in direct competition would be enforceable. Just not forever.
The reality is that most schools don't push these types of issues, because it would make it hard for them to get other coaches if they get the reputation as forcing their coaches to stay, or severely limiting their options. That being said, I think that most coaches and schools respect the inner conference limitations.
The argument of course, is like any other business. Mullen, for example, would have too much inside knowledge of the strengths and weaknesses of the players, coaches left behind, and their schemes which could be used against them in conference play. That's why a limitation of say 4 years would not be overly broad, b/c by then most players would be gone, thus no more advantage.