Interesting theory, and I'm sure in some cases (and I'm thinking particularly of pro and semi-pro tribute bands here), there's certainly an argument to be made that they're making money of the back of established artists, especially where the "tribute" is to a band that's still a going concern. Having said that, if you live in Stockton, it's unlikely that you're going to be able to see U2 without significant expense and travel arrangements, so if a semi-pro tribute band are playing a gig in a pub in town for £6 a ticket, why wouldn't you go? I certainly don't think that they could be accused of taking money out of Bono's pockets!
A more interesting case would be that of bands containing one or more original members. I'm following a lot of negative feedback around the band Yes at the moment, who are touring North America with 3 long-established "founder" group members, and a new keyboard player and vocalist. They're using the name Yes, which they're legally entitled to do, but there's a big backlash amongst even formerly loyal fans that this is nothing more than a Yes "tribute band". I can see both sides of the argument, although in this case it centres more around the vocal abilities of the new singer, but when does a band stop being
the band (just how many of the original Drifters are now touring under that name?!) And, to bring this a bit more on-topic, if you follow the argument about tribute bands being nothing more than musicians wasting talent on impersonation, where does that leave Mr Welch and his Moonlight Shadows?! There's not many people who can say they're in a tribute band to themself
Matt