That's foul as fukk. Non-compete clauses and "for cause" terminations are some of the dirtiest tricks corporations use to control workers. It's even more fukked up because wrestlers are classified as "independent contractors," not employees subject to benefits and full labor protections. How are you going to call someone independent, then still control what they can do when they're no longer working for you? And to think we almost got a nationwide ban on those kinds of exploitive practices.
A wrestlers union would be great, but it's never going to happen w/o outside pressure. They would need a very strong NLRB+lower card/mid card workers organizing together. I left out the people at the top of the card to illustrate how the power imbalance isn't just between management and workers -- it's between different tiers of workers.
The biggest stars, especially those with personal relationships with the bosses (look at Hogan was with Vince), have incentives to keep things the way they are -- they get special treatment, creative control, and big money deals that they'd have to share or justify under a union structure. And let's be honest, they wouldn't go for it.