Because....Astarions not my character? He's an NPC with his own thoughts and motives. *shrugs*
Yep. There's also a matter of playing the character as an origin or having it as a companion. I like the idea of being able to speak as one of the companions, but it's weird when you could say something that would make that same character disapprove. It is a very good point that there are ulterior motives, fake kindness and so on, but in such case, companions shouldn't disapprove of you saying something they could say as well. For all they know, you could have been just as dishonest.