Basically BEG already said what I wanted to say on the subject, Joss might have been an ***hole for firing her for such reason (which I agree with, he was a jerk), but truth is those last minute change did cost him a lot. Being an actress I feel isn't like just any regular job in that department. It wasn't okay of Joss to fire her, but if I was directing a movie/show and had to change a whole season based on one actor, you bet I would be dam pissed about it. It doesn't make it right, but I don't blame Joss for being mad.
I agree, also Joss basically made Charisma's career, she should have had the decency to inform him about something that was going to have such an effect on the show.