It might be in a court, but how many min wage workers are going to have the will and means to spend a bunch of money fighting something like that in court? As such it is probably very effective, even if it doesn't hold up in court.
This is how corporate America works, using the law as a club to beat those that can't defend against it. It is of course very effective when they want it to be, because most of the people they use it against are young, stupid, or both, and none of them can afford to defend themselves in a court. Most capitulate because even if it occurs to them to fight it they have no idea how to go about doing so, and those few that are smart enough to go to a lawyer are then cowed by the fact that even the cheapest lawyer's hourly rate is many times more than they can afford. Unless they get really lucky and someone explains to them how to go about getting someone to take their case pro bono they are at that point lost.
Then even if all this happens, the most the minimum wage worker is likely to win is a cancellation of the contact and wages lost which will probably not even cover their cost in lost time and materials to get the ruling.
Such clauses should not be allowed to exist, with the exception of specific circumstances or industries where the employer can demonstrate a specific rationale for them.
I don't know how you would go about keeping them from existing, you can make them non-binding, and they probably already are, and that would hardly keep a corporation from trying.
The IRS should tax them, that would be effective at reducing them and making companies only use them when they really want to protect themselves instead of this nonsense like Jimmy Johns. Upsetting since i love their sandwiches. Now I have to boycott them.
I don't think you need to tax them, what you need is to make such heavy handed legal shenanigans so expensive for corporations when they lose in court that they don't think it is worth using legally unenforceable contracts as intimidation techniques.