that isnt entirely true. some bands when they started out they are already on mtv, so its their original style and they cannot be considered sell outs. sell outs are like so many have already mentioned: bands change their original style of music from something that they play with passion into a style of music that they know will attract the masses and earn them big bucks.
case in point: A7X. they once mentioned in a metal hammer or kerrang! interview that they wanted to change their style of music so that they can capture more market via mtv and stuff like that.
metallica's change though most would deem it as sell outs, they might not have been given a choice when they changed their style from thrash metal to heavy metal then to hard rock-ish sounds. there are lotsa other factors that could have been at play also. perhaps its cos of bob rock's style of producing the band (note that bob rock produced bands that were more pop metal as well, so it could have affected how he decided to produce metallica). perhaps its cos they couldnt get over the death of cliff burton and stuff like that.
so like i said, bands selling out isnt always up to them to decide.
then again, some bands have never sold out. they never had anything to sell to begin with xD
http://www.youtube.com/watch?v=JYNRhzX6gw8