you can't really link MTV in with the notions of rebellion of rock n roll in the 50s and 60s. MTV was born in the 1980s, the "me" decade and the era of "greed is good", and when the notion of being a "pop star" came to prominence.
i agree with Dan, expecting them to act as anything other than a corporation representing its own interests is an unreasonable expectation - they are only forward thinking and attacking the status quo when there is more money to be made in doing that than there is in toeing the line.
hence, an advertising campaign which uses disguised/censored nudity gets noticed, so makes them money by generating interest... a campaign with uncensored nudity might get them some brief notoriety, but would be unairable on TV in most countries and would ultimately not get them as much exposure, and so would ultimately represent a waste of advertising money.
and that is disregarding the fact that the "censor bars" are used as part of the ad campaign, in keeping the slogans and brand identity on the screen for the maximum amount of time in a way that the viewer can't help but notice.
in short, this isn't art, or rebellion, it's commerce, and the only statement it is intended to make is "watch our channel and help us earn advertising revenue".