I'm a little bit on the fence about Eich's firing as well, even though I wholeheartedly disagree with his stance on same-sex marriage.
But your comparison is not very apt.
This proposition was passed. So by the logic of Eich's
firing the majority of Californian voters would be
excluded due to their political beliefs
Big difference between "voting for" and "funding."
That's why we have secret ballots. So you can vote without fear of repercussions.
For better or for worse, once you have a C-level title, you are seen as representing the company and emblematic of its values. As you move up that ladder, things change.
I can't think of too many corporations where the executives' political activity wouldn't be grounds for some scrutiny.
I'm sure we can come up with some pathological examples where very few people would complain an an executive's ouster, such as an executive who supported openly Nazi candidates and figures. And I'm sure we can come up with much trickier and ambiguous scenarios, such as an executive who donated to a politician who then, in turn, opposed gay rights. When it comes to our two-party system, even with our current level of strife, very few would like to see somebody fired simply for voting for the "wrong" one of the two.