The issue is not voicing an opinion in private -- it's voicing it on an institutional mailing list in his capacity as a member of that institution.
Deciding post-facto smells of favoritism and can never be proven to be fair. It also creates a chilling effect since nobody ever knows what is and is not ok to say, since at any point in time AFTER the fact you might decide it is not ok.
I'm almost certain MIT has an employee handbook that reasonably covers situations like this. If anything, it's a demonstration of favoritism that he wasn't fired in the past.
The beauty of REAL freedom of speech (like we had in 1977 [1]) is that the answer to that is: none
[1] https://en.wikipedia.org/wiki/National_Socialist_Party_of_Am...
Because if i can lose my job over a statement and a mob reaction, the mob makes me unemployable since they scare other employers, i lose my livelihood, and thus my life. If I have no protection from that (enshrined in a law, like california actually has), in what way is speech free? It becomes more of: "speech only as authorized by the mob-du-jour".
That they feel they cannot take on the burden of RMS's stupid comments in addition to their existing scandelous items is not surprising, and should not surprise anyone.
RMS may well have gotten away with making the comments he did, had he been posting on the UCB mailing list.