There's an account here, which quotes me about the law (private employers can punish employees for their political opinions, except in California), but omitted, alas, my opinion that this aspect of employment law in the U.S. is a travesty and an offense to democracy, in which citizens should be able to express their views about matters of public interest without being sanctioned by their employer.  This case is particularly weird because the opinion expressed on Twitter was utterly banal, and yet still triggered his dismissal.  (Note that the editor is also a professor at Berkeley; Berkeley could not, and has not tried to, sanction him for his speech, but he has no academic freedom or free speech rights vis-a-vis the journal.)