Dude, it's a bow. Meant for Killing things. And you made it do what? Fire a "safe" foam projectile at someone? Not the brightest move man, that could actually cause some major harm dude. For example, a freak string snapping occurance could easily damage you, and firing it at someone's chest could injure their eyes, given the innacuracy of a nerf dart. Try it on some of the nerf glasses bro, and see what kind of harm it'll do to the glasses.
To be fair here, arrows are made to kill things, not bows specifically. The original Nerf Bow N' Arrow worked on similar principles as this (except it was spring-powered rather than string-powered, though the difference is negligible, really) and it was safe enough to use.
I don't particularly see this as being a viable, useful Nerf weapon, but let's be realistic here. Suddenly it's okay to make a singled Titan or something that can hit over a hundred feet, but not this? It's not like he's firing darts out of something with gunpowder or any other combustible.