Their personal view of their authenticity is not at conflict with selling the sound track to a commercial is all. I would bet largely it's quite the opposite, they wouldn't be the authentic them if they didn't take the payout.
There are certainly some genres where the audience values this sense of authenticity of course. That and people like Waits who actively cultivate this as part of their public image. Otherwise I wouldn't expect it as a norm.
Personally, I'm still waiting to see Maynard sing Hooker With a Penis on a Coke commercial.