Well exactly, unless they ban all knives, which will make cooking dinner a bit tricky, if criminals want to arm themselves they will use whatever they can get hold of. I'm sure I could make a proper mess of someone with my chef's knife but I'm not going to do that as I don't feel the need to be a murderer.
Also, I'm sure there are legitimate uses for machetes, my Dad had one for hacking down brambles and stuff.