I haven't looked into the enforceability of this kind of contract in particular. That being said, there are arguments in the abstract that I'd apply---essentially, the argument that this kind of contract is against public policy because of those kinds of anti-competitive effects.
Without doing the research, it wouldn't surprise me if the extent to which these clauses were enforceable varied state-by-state. States tend to diverge a lot on these kinds of "unenforceable because against public policy" arguments.