I visit Amazon from time to time and check if any interesting J-pop appeared in MP3 store there (so far nothing except Hikaru Utada who I do not like much). What's notable though, there's a large number of cheap knock-offs and remixes. I always assumed that they were made by agreement with original rights holders: someone bought rights for ancient favourites cheap, re-recorded them and sold as bargains. But perhaps not. For example, they have tracks from K-ON now. Somehow I doubt anyone would sell rights for that...