Because the govt. may have you arrested for marijuana possession if they decide that its not 'safe' to let you have a gun.
They may decide that your 'affiliations' make you a security risk.
Believe me when I say if there are background checks, soon, only powerful politicians and big businessmen will have the clearance to buy guns. They will ensure that anyone who holds an anti govt. agenda gets nowhere near a gun which would exclude almost the whole of DailyPaul from owning guns.