I don't get it, how does this make killing-sprees any harder?

A semiautomatic rifle that has an ability to accept a detachable magazine and has at least two of the following characteristics:  A folding or telescoping stock  A pistol grip that protrudes conspicuously beneath the action of the weapon  A bayonet mount  A flash suppressor or threaded barrel designed to accommodate a flash suppressor  A grenade launcher

Those are basically ALL cosmetic criteria, in none of the recent killing sprees did the perpetrator need any of these things. Grenade launchers are almost impossible to get for civilians in the U.S. this way or another, bayonet mount? When was the last person killed by a bayonet. Flash-suppressors make the muzzle-flash less noticeable, the gun is still as loud as before and huge flames gushing out of your barrel are a Hollywood-fantasy anyway. The folding stock only makes it easier to carry the rifle close to your body, heck - if someone would really want to shorten his rifle, he'd simply saw it off