Because we've been told the opposite for years.
I really hated to see it happen, but I didn't believe it to be against the rules either. I have always read that the last region of a realm shouldn't be able to switch, and that a capital can't secede (it's already a capital!), but that otherwise any other duchy could secede and that even a capital could switch allegiance.
I'm not accusing Chénier of this, but it is a little annoying how quickly players with characters in D'Hara glommed onto this idea of an opportunistic merger. If, instead of assuming that everyone is willing to move heaven and earth for an advantage in a game, they instead sussed out the details IC, they'd have learned that there's a very real disagreement about whether D'Hara was ever really the problem. You know who did bother to find out? Vellos.
