The reason that markets such as Japan and the UK tend to get bonus tracks is because often times, for multiple reasons, it is easier and cheaper to buy imports (say, North American imports) than it is to buy the domestic version. By including a bonus track (i.e. Holy Joe, The Ground Beneath Her Feet, Fast Cars) the domestic market is encouraged to buy the domestic product. It's simply economical and frequently bands, even bands as big as U2, have a say in which extra song is included, but not as to whether or not an extra song is included in the first place. It's the record companies' decision.
james