Because, your know, we gotta make the world safe for everyone, even fools and/or drunks.
Via FERFAL, I bring you this knife from Britain. Is it any wonder the British Empire is no more, with thinking like this? Soon, I am sure, this will be the only type of knife allowed on Britain, with forced exchange of your current knife for one of the new design. Read the website, realize that it is designed to prevent you from being able to stab or cut anything that is not food.
I have indeed cut myself with a kitchen knife, (but I didn’t like the resultant pain and bleeding, so I try not to do it anymore) but if the referenced knife will not do to stab anyone, then I would find another tool for the job, should I be inclined towards homicide. If you are too stupid to operate a knife, (or too drunk, or whatever) then you deserve to cut yourself. You’ll either bleed to death eventually (cleansing the gene pool) or learn not to cut yourself or to operate a knife while drunk.
How safe is the world supposed to be? Can the state remove anything that could be a weapon from everyone and still allow us to do anything? Or is that the point? (or lack therof).