I don't know exactly what she said, either, but I do think a company has a right to fire a spokesperson for numerous reasons. They choose a spokesperson because of their image, and, if that image changes or becomes controversial, it can reflect on the sales of their product.
It doesn't matter if Slim Fast is pro or con Bush, they are most likely concerned with not alienating members of either political party, because they are all consumers.
Of course, it will come down to legalities, meaning that there has to be an ironclad clause in Slim Fast's contract with Goldberg to that effect, or she can sue. I'm sure such a clause exists, though.