The usual explanation in these threads are "chargebacks", but come on. Payment processors could deal with chargebacks and disputes just fine. They suck for the seller, not the credit card company.

What US companies are afraid of more is PR and regulatory risk. Zelle has no chargeback process, but still bans the sale of automatic knives, fireworks, ammo, and firearm parts. Venmo bans a nebulous category of "products that present a risk to consumer safety". You better not be buying any vintage lawn darts for your collection.

The chargeback rate on knives or firearm optics is probably not any higher than on anything else. What's higher is the likelihood of a headline along the lines of "kid dead / injured because of Paypal". And so, we end up with digital payment processors as the arbiters of morality.