Presumably from your description those people believe he was fired. He was not. He clearly stated that he had resigned; his choice, his action.

I am unsure whether people so often assume he was fired because that's what fits the narrative they want to push (bullying intolerant liberal witch-hunting mob action... choose from any of the above, they've all been used), or because it's next to impossible to argue that he was wronged if he resigned.

(the difficulty being that making someone feel unwelcome through overwhelming social pressure can't be labeled wrong in the "Eich was wronged" worldview, because then the overwhelming unwelcoming social pressure of Proposition 8 comes into play as a "well, he did wrong")