Funny, plenty of people get killed by criminals with assault weapons all the time in Detroit, where I spend quit a bit of time during the week; and yet apparently it only becomes a problem if I, a 'non-criminal' can own one.
And of course, that is the huge fallacy in the liberal mind. A: that assault weapons (whatever that is) in the hands of law abiding citizens is a threat. B: that if you don't allow the sale of assault weapons criminals will sit at home and find other things to do besides commit crimes with guns.
My guess is that it has somethign to do with the predicted "flooding effect" upon the streets. I've already surrounded my house with sandbags to keep the flood of ARs and AKs at bay. I predict it will be like a salmon spawning run -- I'll just hang a net out in the street and harvest a few hundred rifles tomorrow.