Maybe they think that if the games remained in Japan, there is greater opportunity for them to recirculate the local market again. Whereas when foreigners buy them they are gone forever to the land of being traded on eBay instead.
Similar to how if you bought a game at Gamestop. There is a chance that game could end up there again some day. Either now or after you dump your old box of games and DVDs on your little brother. Taking too many out of circulation, over time, dries up the ability to backfill sales.