If you stopped coming to your job because you didn't like your position in the company, I'm pretty sure they'd fire you.
But in the wacky world of wrestling, wrestlers can just decide to go home and they keep getting paid and remain under contract despite not actually doing the job they agreed to do when they signed a contract with the company.
Sounds like a pretty sweet deal to me.
That is a bit of a ridiculous oversimplification. WWE wrestlers are not employees, they're independent contractors. They sign a deal for (x) years and (x) shows, and when that contract is done they are free to go elsewhere or re-sign. In the case of Punk, he had already fulfilled his end of the contract, so he was absolutely under no further obligation to wrestle any more for them or do anything else. Was it the ideal way for him to deal with his frustration? Probably not, but from a legal standpoint he was 100% in the right to leave when he did.