A frequent claim made by opponents of marijuana policy reform is that hardly anybody is ever really arrested for low-level marijuana offenses. But like most prohibitionist arguments, that's a lie. In California, where marijuana possession was “decriminalized” in 1976 and medical marijuana legalized 20 years later, the state Department of Justice reports that law enforcement conducted a record 78,492 marijuana arrests in 2008. About 80% of these (61,366) were for mere possession – not sale or cultivation. The…