Sunspot has played in the Hellfire club in fact he ran it the Angers working with a man who he called at traitor would not be big leagues. Sam's goal has always been to be in the X-men for him the Avengers would be minor leagues. Both are tremendously loyal to mutants and the Avengers is no place for anyone that is loyal to mutants
They don't exactly give parades for the Hellfire Club either. Hell, a lot of people probably don't even know Sunspot was a part of the Hell Fire club.
And I think Sunspot stopped considering Logan a traitor the moment he switched sides and fought alongside the Avengers. He was in that group of X-Men that apologized to the Avengers for their actions, and sought to work with them to correct their mistakes.
I think Bobby's a character that does want a bit of the spotlight. And I do think there is an in-universe perspective that the Avengers are the big leagues. He probably grew up idolizing them as heroes.
As far as Sam goes... I don't think he necessarily cares for being A list. But I do think he'll join simply because he was asked. It's not like he or Bobby for that matter are really doing anything else.
As far as the Avengers being no place for loyal mutants... you make the mistake in thinking the characters believe that simply because you do. Clearly that's not the case.