Well, if this is how everything has gone down, then it really doesn't surprise me that Disney or the Muppets Studio would fire Steve on those terms. Why? Because this is something that happens all too often in the entertainment industry: somebody "gets out of line," (so to speak), gets too complainy, too nitpicky, too "demanding," they can face termination for it. Show business can be really grudging that way, this is why more oftne than not, actors/performers/day players/what have you are too scared to speak their mind the way Steve apparently has been, because they know this will get them into hot water, and they could lose their jobs as a result of it.
With that being said, I kind of have to side with Steve on this one: I've made my opinions known for years that I feel the main problem with recent Muppet projects has been that Disney won't bring in actual Muppet writers to work on them, so I can totally understand why Steve may have had issues with the scripts for the new show, and I can totally understand why he was nervous and apprehensive about the 2011 movie, and I can understand if he often "fought" with Disney about the direction the Muppets were going in.
At the same time, I can understand why other parties were probably becoming increasingly annoyed with Steve's nitpicking and complaining, however it's a tough pill to swallow seeing these accusations from not only Disney, but the Henson family as well that Steve's behavior goes beyong nitpicking, but that he's also been demanding, stubborn, condescending, and would apparently "attack" other performers and Henson/Disney employees. Nevertheless, I have a feeling if Jim were alive to see this situation unfold, his reaction would be . . . to just sit there quietly and avoid the ugliness as much as he could, and probably even console Steve with a big bear hug like he did for Richard Hunt after David Lazer chewed him out once. I know Jim tried to instill this sense of human compassion into those who were part of his life, including his family, which is why this makes Cheryl and Lisa's comments hard to swallow (Brian to a lesser extent, since he seems to be "meh" about the whole thing so far).
Like Drtooth and others have mentioned earlier, this is really turning into a "He Said, She Said" scenario, and really the only consistency with the story is that Steve was fired for his conduct and for being too nitpicky . . . Steve's account that he was simply trying to fight for the integrity of the Muppets could be true, Disney and the Henson family's account that he was becoming a pest and nuisance could be true.
Either way, all I can say is with the recent happenings in the Muppet world, from the deaths of Jerry Nelson, Jane and John Henson, the resignations of Kevin Clash and Joey Mazzarino, and now the termination of Steve Whitmire and the scathing things Disney and the Henson family are saying about him . . . I really believe this would all be way too much for Jim himself to handle.