It's true that while I don't think there's anything inherently wrong about "selling out," it's also the case the rock bands are also brands in and of themselves so turning half their catalog into commercial jingles for canned soup might negatively impact their brand. The issue isn't "how dare they sell out to Big Soup!!!" it's that maybe their fans don't want to be at their concerts thinking about dancing soup spoons.
Like other commercial enterprises bands have a brand to protect, and associating that brand with other things might have negative effects. But that's a different issue than objecting to them making a buck.
Post inspired by email from Brendan, who used to play with this band.
who apparently once sold out to "Big Felicity."