Then you have a German freak out over even talking about Nazis.
Almost every country has some hang up or another. Suggesting that 'Americans' have a fear of nipples is short sighted. Every culture has something that causes a collective freakout.
It's why, for example, they are often puzzled when abroad and note that "some people don't like us Americans" and cannot find out why.
I have noticed this particular way of looking at the world with many Americans when working in global collaborative tech projects - it's a kind of imperialism - a built in sense that their way of looking at the world is the only way. It seems to be a common thing, and not everywhere. It also has the characteristic that those with that mindset are unaware about it.
There are many exceptions of course, and the most successful people are able to work with people from other cultures. But I believe this is what the article and mgleason_3 is trying to say.
If you'd like another metaphor to explain the theory: Privilege. Americans have a privilege as the "top" culture which makes them blind to others below them.
Americans tend to live near other like-minded people and are afraid of those with other views - both in our own country and outside.
Facebook pushes this filter bubble which we gobble up. It's not an accident, and it has a scientific basis. It's also better for advertisers.
Regarding sex and violence and censorship - I agree it's a little odd. The only thing I can think of is that there are more laws and regulations regarding sexual photos than violent photos. Also, our media also has a history of pushing violent stories. If it bleeds it leads.