When this comes from a musician, I'd say the term is just sour grapes coming from those who have not had the level of success as those they are criticizing. When it comes from fans it is just a lazy way of saying "I liked your other stuff better so why didn't you keep making that stuff?" But of course, when you stick with your formula, those same fans will be quick to say "your new album sounds too much like your last album!"

If someone plays only what they want to play and refuses to change even when there is the possibility of commercial success then that is fine...for them. On the other hand, if someone is playing their favorite bluegrass and a record guy hears them, is impressed with their technical ability and offers them a chance to record some hip hop and the band decides to go for it because it might get them the break they need, again that is just fine.

There is no "selling out". Only doing what you like for whatever reason you want to do it!