US citizens and permanent residents can change jobs freely, drop out of the work force, go back to school, transition to a new career, and so forth, all without jeopardizing their right to live and work in the US. While H1B workers do have some rights to change jobs, their mobility is considerably more limited.
You don't have to be especially cynical to believe that the very one-sided nature of the "at-will" relationship here, along with the high stakes for the employee (if you can't get a new job with a new sponsor, you will have to leave the country), is huge part of the appeal of these visas.
So, was it legal for Disney to fire them? Probably. Was it legal for Disney (or rather, its contractor) to replace them with H1Bs? Probably not.
It is incredibly shitty and absolutely against the spirit of the visas, but it's not against the letter of the law.
It might be because I have a low tolerance for clickbait headlines and articles.