I think the reason many Japanese people mainly listen to American music is thre is a lot things from which you can get information about them such as Music Stores, TV programs, etc,,,
Have you ever been to the CD shops in Japan? Every single shop has at least popular American Music things at the times, if you go to big ones, you can get almost perfect range of American music there, but not much music from other countries except for UK.
BTW, when my Australian friend visited me we went to a rental shop and he was really surprised at the fact you can borrow CDs from them. It seemed like in Aussie, it's very unusual to borrow CDs from rentals. How about other countries?