I've been thinking about this, about selling out, about artistic integrity, and about the counterculture, which I believe is where this whole concept of selling out came from. I'm reminded of a line from a Glenn Fry song (or was it that other guy from the Eagles?) "Saw a Dead Head sticker on a Cadillac..." and I think that sums up the whole thing.
The counterculture icons were the Dead, Jefferson Airplane, Bob Dylan, Arlo Guthrey, and many others of that Woodstock era. Names like Timothy Leary, Alan Gingsberg, Jack Kerouac, Hunter S. Thompson, Jerry Rubin, Abbie Hoffman (co-founders of the Yippies)...
Led Zepplin was never really a part of that counterculture movement. In fact, I'd say that they, along with Black Sabbath were part of the anti-counterculture. While everyone else was singing about peace and love, these two started out with something so heavy and raw it caught the world offguard and started a whole new genre of music.
I understand what selling out means, and though I don't like seeing songs I grew up with that spoke to my teenage angst trivialized by making them into ad jingles, I also realize that Led Zepplin probably wasn't part of that whole counterculture crowd.