Same with Apple not allowing nude pictures on their iBook store.
And when you start providing that sort of adult content, I imagine (in the US at least) different rules begin to go into play. Just to be friends with someone, you'd need to prove you're 18+; because, as you defined it, some friends-only photos could be 18+. Then you've got other issues, like what it makes facebook look like. When it's possible to have pornography on a website, it changes the demographic that's willing to go to that website. It certainly wouldn't be a large percentage of the world, like it is now.
Showing pornography also affects the kind of people that are willing to advertise on your site, and so you may lose money that way. Mature images open a biiiig can of worms.