IANAL, but it is illegal for employers to use the threat of firing to compel employees to take (or avoid) a political course of action.
Furthermore, it is illegal for employers to punish employees for speaking up about issues the employee believes affect working conditions (which was largely the premise of the employees essay).
Google side-stepped the issue by firing him for statements which may (or may not) violate anti-discrimination policies that just so happened to be in his essay.
They left largely unchallenged (and it seems to me, rather reinforced) the central complaint. Namely that Google was fostering a politically stifling atmosphere.