The real issue is money as speech.
I'd wager that most of those against Eich's position would not begrudge anyone freedom of opinion, or private speech, or even public speech so long as such opinions or speech did not elevate to action. On the other hand had Eich fired a LGBT staffer based on their sexual orientation, not only would he be in the wrong legally, but I think even those carrying the banner of free speech in this case would agree that his promotion was wrong.
So the real question is: by donating money to the Proposition 8 cause, was Eich exercising his right to free speech? or was he taking a discriminatory action?
Of course, you may note this is exactly the same issue in question with the Citizen's United decision. I think this larger question, is money speech or action, will be one of the most important questions of our time...