I see random offhand unsubstantiated online comments here and there including here on HN, that 1) chargeback rates of porn, games, and digital contents are significantly lower than anything else, and 2) credit card companies already charge higher fees for porn despite that.

Combined with prevalence of the suspiciously well standardized "because porn users and gamers chargeback Steam purchases way too often" canned responses, I think it's just an excuse, if not "the" excuse somewhere - like the basis for using incorrect data for internal risk modeling or something like that.