You can't buy weapons on eBay, either. CVS no longer sells cigarettes. Famously, K-Mart stopped selling ammo after Michael Moore went after them. That said, were Visa and MC to refuse to do business with anyone selling any kind of gun, it'd make the PayPal/Wikileaks embargo look minor.
This is an interesting take, although it does raise a question about what else Visa could block and what kind of activism we'd be okay with them dabbling in. But good lord it grinds my gears whenever someone uses the term "assault weapon" and I know that's the wrong thing to focus on but please at least have some familiarity with the subject you are writing about. "AR-15 style semiautomatic firearm" is better, but later the author uses the term again and even references "assault rifles" in a later paragraph. Pedantic, perhaps, but Wal-Mart definitely never sold those.
I like that a conversation is happening. It needs to happen. There are solutions out there. I don't think the solution in this article is the answer, but it's an angle I hadn't considered.