You're absolutely right. Why does anyone need a knife with more than 3 inches of blade length? There is no sporting purpose for these assault knives with blades longer than 3 inches.
The knives probably also had high-tech polymer grips that allowed the criminal to stab people from the hip, further increasing the knives' lethality.
And it will probably be found that the knives had several other "military" features, such as a fuller for bleeding out the victims, a reinforced point for stabbing, crossguard to make the stabber invisible, and a full tang for prolonged periods of use.
These kinds of knife belong on a battlefield, not on the streets of America, or in our schools. Think of the children!
/HEAVY, dripping sarcasm off
Maybe the magic wore off of those special "No Weapons Allowed" stickers and window decals.