Confused by decency standards

Yesterday on random Iowa radio stations, in the middle of the day, within five minutes of each other, I heard Tom Petty sing, "Let's get to the point / Let's roooll another joint..." and Snoop Dogg sing, "I got the rolly on my arm / And I'm pourin' Chandon / And I roll the best ____ / Cuz I got it goin' on."

Now, isn't "joint" just as bad for our precious, innocent youth as "weed" is? Is it more dangerous for a black man to sing about the herb on the radio than a good old white boy? Or do different stations have different fears and standards (but what creates those)?

Despite that, hip-hop music appears to me to be the most subversive popular art form around. Kids all over the country -- the world, perhaps -- are going around singing about being pimps and ho's and gangstas and killing cops and selling drugs. That's fine, apparently, but Janet Jackson's nipple provokes national outrage, congressional hearings, and FCC fines. As Chris Rock said in his HBO special, shaking his head, "A titty on a Sunday afternoon! A TITTY on a SUNDAY afternoon! And a forty-year-old titty at that."

This confuses me. TV is clearly held to a different standard. Seems to me it goes like this, in order from most restrictive to least in terms of censorship and decency "protection":

Network TV
Radio
Cable TV
Magazines
Movies
CDs and MP3s
Books
The Internet

But if the idea of decency is to protect "the children," why is the children's favorite art form (music) allowed to be the most wildly subversive? Tracy suggested to me that it's the advertisers. That in the preceding list, advertisers have less influence and less to worry about as you go down the different art forms. Maybe that's it. We don't REALLY care about protecting children, we care about protecting the money that supports the particular media, and we couch that in moralistic public blather.

But also, is the visual image more sensitive than the audio image? It must be. You have to be 17 to buy a ticket to an R-rated movie, but you can hear what would be X-rated music (see "Work It") without any form of ID (and the "explicit lyrics" sticker is a great way to find it). Was it Plato who distrusted music and worried about its stirring the passions of the masses? Yet by and large music gets a pass in our culture. Is it because parents don't have any idea what hip-hop songs are about?

And yet -- why censor Snoop and not Petty?