Point out the problems caused by marijuana during the thousands of years of unregulated use. The regulatory environment is the main driver for the problems caused by most drugs in the modern world. If it weren't illegal to use/possess/sell, people would largely not be stealing or killing or doing other antisocial things in order to do so. I'm not really a fan of decriminalization as a panacea to societal problems around drugs, but criminalization was the cause of most of those problems.