Not all opinions are the same.

Advocating for open borders and basic income are fairly straightforward political opinions. So is being a member of the republican or democratic party, or saying you support lower taxes, or even that you voted for Trump. Had Stallman resigned over reactions to calmly expressing these type of opinions, the fallout would be very different, and I suspect most people would say something similar to what you've said, and side with him.

But if a person repeatedly says, in public, that our definition of pedophilia as a necessarily-bad thing isn't right, and that people are being too hard on the billionaires who recently got in trouble for this -- even if they do so calmly and coherently -- they are espousing views that many people believe would lead to actual harm to actual human beings. The same would be true for someone who openly supports fascism, or calls for the deportation of Hispanic-looking citizens (I want to avoid a straw-man here, so to be clear I am absolutely not saying Stallman supports these views; they're just examples).

In that case, don't other people have a right to react negatively to that?

As for the consequences of that reaction, that is somewhat proportional to the person's position. If that person was a gas station clerk who, outside of work hours, had posted something on a forum, then we'd again be having a different discussion. But the positions of President of the FSF and Visiting Scientist at MIT carry a lot more weight. Putting someone in these positions who not only holds but eagerly volunteers these types of views is seen as an implicit endorsement of these views by the FSF/MIT -- _especially_ when he chooses to broadcast these views directly to his work community, directly in defense of someone at the center of a recent scandal.

> Firing people for [their] views ... is certainly violation of free speech and thought

Stallman is free to _think_ what he wants. He's even free to _say_ what he wants -- he was never censured afaik. What he is no longer free to do is to continue doing so from the position of President of the FSF or a Visiting Scientist at MIT. Should an institution (such as MIT or the FSF) be forced to protect its personnel from all consequences for individually sharing _any_ opinion in any public forum? I don't think they should.