Lougher identified his pot addiction to the army recruiter and was denied enlistment by the army. You cannot legally buy a weapon if you are addicted to controlled substances. Loughner went to the army to enlist and freely admitted that he was addicted to pot. There's a strong reason to consider Loughner's behavior as questionable.
Finally, AFT 4433 where it asks if you are addicted to illegal drugs Loughner checked "No".