I think Elon's a tool, and this is a bad move, but to think someone should be protected from consequence of what they express is absurd.
It's his company, he makes the decisions. The market should respond if that's a big enough deal, and I'm 100% sure that's starting to happen (though it's exceedingly slow in the space domain).
I have to assume they knew when they penned the letter that they would find out whether their leader could take criticism and help them make a better company and product together, or react immaturely and let them know that their time would be better spent elsewhere. Seems they got their answer.
In any case, yeah, Musk owns the company and has the right to fire people for criticizing his business decisions. Bold strategy, we'll see how it turns out for him.
I think there's also this game that gets played now where internal dissent tries to whip up external dissenters to get their way within companies. Leaders need to decide if this is happening or not and act accordingly.
This is cringe worthy. They knew exactly what would happen and expected to rally support online. I can't say with certainty what the ultimate goal was.
This is common and has been common for hundreds of years.
Burn the boss, lose your job.
It isn't rocket science.