Explanation Please

The Boston Marathon bombing suspect is being charged in civilian court with, among other things, conspiring to use a "weapon of mass destruction" against people.

I'd like to know how a pressure cooker bomb which kills three people is a "weapon of mass destruction" but a semi-automatic rifle which kills 20 kids isn't. [UPDATE:  Or killing four people with a gun, which doesn't make the news like a marathon bombing]

Spencer Ackerman touched on this a while ago, and he's right.

What do you think?