Depending on the time that the fork would occur (between difficulty adjustments) you could say that they aren't allowed to. The chain becomes rendered useless for a longer time because of the other majority. It is clear that we can't get everyone on board on a single proposal (and thus 100% consensus is impossible), but nobody sane would recommend a split. Essentially the people behind the fork do not care much about the whole industry and especially not about the users, they just want to take over.