Although "selling out" has a proper meaning, I think it is typically applied to bands that people don't like. In one sense, it is impossible for a commercial band to sell out, since the point of putting out a CD or download or whatever with a price tag on it is to make money. Many people seem to forget that, and imagine that somehow their favored band is "pure" and is doing what they are doing irrespective of money. But anything with a price tag on it is about money. It may also be about other things, too, but it is about money regardless of whether it is about anything else.
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.
