I buy weed with a Visa card all the time.
Do you mean you can't buy weed in places where it's been only decriminalized but not fully legalized? Decriminalized means it's still illegal but the government just decided to stop enforcing the law. It's not actually the same as legalization, and yeah it results in all kinds of stupid situations. And yeah, a big company isn't going to engage in something that's technically illegal even if the law won't be enforced.