Partly it's down to the Geek Social Fallacies thing, I think, partly down to the myth of fannish tolerance, and partly down to the lack of a formal, overarching organisation. Different concoms have different views, and are variable about how they feel about external pressures.
There is also the legal side, which I can't speak to. I suspect major_clanger will know, though. Cons are private events, technically, rather than businesses, and the rules are different, I think. People have been removed and had their memberships refunded in the past, and one person has been refused a membership that I know of. But it's tricky.