So that's kind of missing the point. First as I pointed out you don't actually have to buy anything to see the explicit preview images, so Steam is arguably in violation of those laws. Secondly the issue is that the Visa and Mastercard contracts require companies to be in compliance with local laws. It doesn't matter whether someone is using a Visa or Mastercard to make the actual payment if the purchase would technically be considered illegal (which it arguably could be in some states/countries under the new super strict porn laws).

At the end of the day this boils down to a) terms of the Visa/Mastercard contracts, and possibly b) new anti-porn laws that are putting an onerous burden on services to collect customers IDs in order to prove age. This isn't a question of common sense, in contract law (and law in general) it's about the letter of it and not so much the spirit. Yes, it stands to reason that if you legally own a credit card, and you must be at least 18 to own a card, then you are obviously 18 or older. However that doesn't matter at all when the laws are written such that services must validate age using a photo ID. It also does not account for stolen credit cards (never mind that that's a far more serious situation than the possibility of under age kids seeing some naughty pixels).

This whole situation is stupid and Visa and Mastercard clearly need to make some changes to their terms and conditions, but until they do from a legal standpoint businesses like Valve and Itch.io have their hands tied.