Not the least of which is because, in Japan, "fuck" is weird. Here in the US, we use it as such a daily crutch of expression, that we forget just how different it may sound to other cultures for whom English isn't their first language. Thankfully, there's a book that can help them with this issue.
You might be aware that there is currently a book on sale that will teach Japanese folks how to use the word f*ck properly. It is comprehensive and hilarious and comes complete with audio CD with lessons on using others words like sh*t, damn, and hell. The last two probably used as filler because they’re not f*cking swear words anymore.
Anyway, the wonderful gent who runs Abroad In Japan bought a few copies of the book and put it to the test with some natives in Japan and the above video is the result. It seems like a lot of fun.