I believe Harper was released from his contract, which would mean that he is not getting Paid for the contract, which triggers the 90 day no compete clause. Obviously you are not going to release a wrestler from their contract and still pay them, if you are going to be paying the wrestler, you are going to job them out until their contract expires because you would be out the money anyway.
PAC is an interesting example. As far as I know Pac refused to wrestler, which meant his contract got extended for everyday he didn't wrestle (but I would assume he wasnt getting paid). This is kinda how wwe extends contracts for wrestlers who get hurt.
Legally if a wrestler refuses to work they have essentially broken the contract and wouldn't get paid. They would likely have to pay the company. I would imagine that WWE has structured their contracts so that wrestlers can refuse to wrestle but their contracts are paused until they wrestle again.
Either way the game does need a better way to handle workers that refuse to wrestle as no real company would finish paying the contract.