Because the Dutch military told them to! It saves about a pound (iirc), makes then a bit cheaper to order en masse, and gives them a handy place to stick a QD sling swivel mount. Also, while Colt Canada may eventually make a commercial IUR, I was told that if they do they probably won't come with that flash hider nor the grenade launcher recoil lug on them. So this may be the only chance to get those features.