So you don't agree that illegality or punishments should have to do with the damage something is capable of causing (or damage vs. effort - e.g. you can kill someone with a knife as well as a gun, but not from a distance)? I don't think Confundus is capable of doing as much direct damage to a person as Imperius. (Collateral damage, surely - but that's the case with a lot of things which are legal for civilians but known to be dangerous.)