Can you buy a gun with a credit card in the US? I presume yes. Why would other weapons be different?

Yes, you can.

Because Visa said so presumably, which is the issue at hand.

Becuase guns have higher protections, more stringent seller regulations, and advocacy groups. In some localities, things like swords can be illegal to own. The dealers generally don't need a special license. With guns, you have an FFL which is heavily regulated. So as a payment processor, there's a greater chance that a merchant selling a sword might be violating the law than an FFL selling a gun. Then the advocacy groups for guns are much more active than the ones for knives and swords.

you could have just said advocacy groups and be done with it. That's all that matters, someone who can yell loudly when they try to ban something. There's regulations and protections around an 18+ anything as well.

Thankfully, politics is now a arm of corporate policies so theres really no real concerns about fascism.

Don't forget the </sarcasm> tag.