I think it's because they really pimp those singles. They will release about 5 singles before releasing the album. In some cases the songs are released over half a year before the album sees the light of day so they record a whole mess of extra tracks to release as well. Money!
Yeah but my point was that most of the B-side tracks are better than the singles they actually come with.
Like Plastic Tree, for example. I think all of their B-sides are so much better than their singles. They actually released an album of all B-sides, and it's probably my favorite of theirs. But, in their case, they recycle songs like crazy! I guess dumb japanese fangirls must buy everything.