Who ever said the law was supposed to make sense? Look how many places outlaw automatic knives. As if 95% of the knives on the market weren't designed to be flipped open one handed. Somehow, that spring makes them SOOOOOOOO much more dangerous.
When Colorado banned "high capacity" magazines, they exempted ones that you already owned. The reasoning? Because according to the sponsor of the bill, people will shoot the rounds in those magazines. Apparently, in their world, magazines are a single use item.