The problem here is not so much someone stealing stuff or making money I think (e.g. if a rogue company puts it's game for sale without approval, after Valve finds out they're going to get sued and probably have to return all earnings); the problem is anonymous vandalism. If the system is really bad some people can wreak havoc stealing sensitive data, unduly charging people for games, polluting the front page with fake games, subtly undermining competitor games, etc.