It's the same old shit - weed makes people crazzzzzzzzzy and turns them into criminals who can't control themselves!!
No, weed makes people tired and hungry and giggly. Previously existing cases of mental illness or disposition to criminal behaviour turns people into crazy criminals. If someone robs a bank when they're stoned, you better damn believe that they'd do it when they were straight too.
I actually bet weed has prevented more crimes than it's directly caused - I know if I was intent on mugging some people down on the corner and smoked a joint before I went, I'd end up breaking open a bag of Doritos and start playing Xbox instead. I'd much rather be around a stoner than a drunk any day.