Many republican politicians have demonstrated contempt for low income workers, so this would not mean "regulating employment contracts" but rather saying that you have to pay for the full extent of a contract.

This framing makes it easier to maintain the image of hating unions/perks/whatever, but is also simpler to implement: there are legitimate reasons that you may not want high level/executive members to not work for your competitors. Whether or not you agree with that is moot: there are enough lawyers in the legislative branches to believe that it's necessary. A law like this would not be an outright ban, but rather would result in companies reducing their use of NCs to just those employees where the information that had was actually valuable.

Currently NCs have no downside for a company, so there is no reason you would ever not use one: They allow suppression of fair market wages, and literally have no associated cost. This would mean that there is: You want to put a 12 month NC on your janitors? Well you now have to pay for a 12 month vacation. Immediately - NC clauses don't say you have to work for X months for the NC to come into effect. If you work for a company for a week, you are bound to an N month NC, and so a law like this would require a company pay for that time - again, making a company actually think about whether an NC is actually necessary.