[newninja]

I'm assuming its because things you can do with family, animals, children, etc. is not necessarily illegal, but does not want to be done. For example incest may be illegal but messing with your brother's/sister's underwear is not, but people don't want to do those dares that involve anything to do with their family especially in a semi-sexual way. As for acronyms, I assume clarity is important in limits, just like you don't abbreviate safety warnings.

I can see where you are coming from, but there are things that are more specific than just being illegal.