BrainWorms

It's access to money you have already earned, is in the bank, you own, and Mastercard is saying you can’t spend it on a particular thing.

Strictly speaking, they're saying that they won't facilitate the transaction themselves, presumably because they're concerned about legal exposure from processing transactions for goods that are illegal under federal law. State law may not be restricting your access, but federal still is, at least on paper, even if it's largely de facto unenforced.

Mastercard makes money every time you swipe a card. They're not going to cut off their own income unless they think the cost of not doing doing so is higher, such as legal exposure.

I agree that I don't love how much control a few private companies have over the financial system, but at least with this case, I think the real issue is Congress for not getting around to legalizing Marijuana.