So over the air networks are regulated by the FCC. Nudity, strong language, etc are all regulated. So they have legal limits they have to comply with. However the cable channels, they can do as they wish. Why then are only the premium ones the only ones with (more than a little) nudity? Ad money. The networks all have standards and practices divisions to work with the creators of shows to keep things such that advertisers are happy, and to work with advertisers to keep them happy. Advertisers worry about their brand being associated with certain things and so the networks have to keep them happy. I'm not saying the advertisers are being sensible, but that is how it is.
This is also why some shows can get away with more than others. If it is a big show, that lots of people watch, advertisers will be more willing to STFU and deal to have their spot played during it.
For that matter, the Internet gets similar things going on with ads. For example Fark used have "boobies" posts/threads with links to naked women. Generally pretty tame and not a large part of their content. However, advertisers kept bitching and some refused to do business, so Fark spun that content off in to a different site and now doesn't allow it on the primary domain. There was nothing legally keeping them doing this, just ad revenue.