It's not MasterCard's fault, it's the federal regulations against marijuana that make it illegal for MasterCard to process these payments. Blame the federal government, specifically the DEA who've become their own legislators and enforcers (they've usurped the original checks and balances of the Controlled Substances Act, it's pretty fucked up actually).
Bottom line: War on drugs has been a catastrophic failure of epic proportions and yet we haven't gutted and dismantled the sole agency that's gotten fat off of it and operates with barely any oversight.
Oh don't get me wrong, they're an evil company like the rest of their ilk. I was just clarifying that in this specific context, they're not the ones at fault for once.