Because having a shotgun with 12 bullets is extremely over powered? Don't believe me? Go play Call of Duty.
Besides the fact that shotguns don't use bullets, you're missing the point. He's talking about the shotguns in Halo (and pretty much every other modern FPS games) where the pellets vanish after going about 5 feet.
Game collection: http://postimage.org/gallery/1tqkmeku/