I think lots of people posting about this have never worked for a major corporation or never worked in a management context. They simply do not have the inclination to white knight for an employee, rightfully or wrongfully. If you want to keep your job, you can't be stirring up shit (even if you're in the right) while also not be meeting all your employer's expectations to the letter.
She was becoming a headache, they allegedly found legitimate reasons to get rid of her, and they acted upon them. This isn't even a case where you can allege what they did was employment discrimination and pretext for firing her, because they could have legally fired her anyway just for tweeting about the stuff.
Are we forgetting that she advocated for the legalization of child pornography? And she works at a company that caters to minor children? What company in that context would be okay with her saying that? Imagine if the PR manager for Fisher Price tweeted that out. Frankly, I'm shocked she wasn't fired sooner.
When you're an employee and you hold view outside the norm, you need to decide what you value most and be careful what you say. That's just the way of the world.