why are band members not allowed to say certain stuff?

[Obi Wan Knievel]

Well, because certain stuff is completely forbidden in certain places.

No matter how badass a band is or wants to be, there are rules that need to be followed. For example, WalMart won't stock albums from a band that says F or other dirty words too much. WalMart owns the shelf space, so they get to make rules like that. Other times the law steps in, because saying F over the radio is not allowed in most places. MTV and MuchMusic won't air videos that have certain images in them.

If the band is playing at a family-friendly event or venue, they have to limit their language and activities accordingly. It's OK to say F in a bar full of drunken headbangers, but in other settings it's against the law.

While bands like to say they're rebellious and badass, they still have to worry about social norms and regulations. It's all about marketing to professional bands (that's why they do it for money), and the smart ones are the ones that play by the rules.