To be fair to Selenia, s/he's roleplaying the character. She was getting letters from the start of the rebellion from both sides, and since her ultimate goal was getting Magnus to step down, once he agreed there was no reason for her to join the rebellion. Don't hate the player, hate the game.
I don't get the BS comment either, maybe because without Dodger the rebellion lost a lot of it's fire.
Likewise in Aramon's case, he didn't mind Magnus but found the rebels annoying, like whiny little brats.
I find the loyalists to be snobs with their heads up their asses, so the annoyance is mutual.
