If you are taking electronic payments you are extending credit. The rails (currently) require it.
Cash by mail would be either the merchant extending credit to the purchaser if they allowed the game download before receipt or the purchaser extending credit if not.
You're speaking out of ignorance here. Steam has a long-term relationship with its customers. They can and will remove a title from a user's library if the payment is charged back, as well as placing restrictions on the account.
I'm a long term customer of Steam _and_ work in the payments industry. In the _easiest_ Steam payment there are 7 different parties extending or receiving credit from each other. Most of those counter parties have no relationship with each other at all (I certainly don't maintain close ties with the game publishers bank), do you?
The rules that are being invoked around porn exist to protect different parts of that chain, from each other.
The argument about Steams ability to revoke the good in question doesn't help with that. What it does is provide an alternate path if Steam wanted to extricate themselves from that chain of 7 counterparties. Steam could stop being involved with all those other players and become a payment network unto themselves. They could demand ach (or cash in the mail as the parent suggests). Give the game with the understanding they would rapidly revoke it for bad customers, and limit their exposure to the publisher by giving them very delayed payment terms. Of course, then the customer is taking on more risk and losing choice and Steam has to account for that in their sales numbers.
By this reasoning every transaction involves credit, since there's a time period between me swiping my card for the apples, and me walking out of the store with the apples, during which time you could tackle me and take my apples. And if it's a word that applies to every transaction, then it's meaningless.
Every transaction _does_ involve credit risk (specifically counter party or settlement risk) but the amount of it changes based on the difference in time between when payment happens and the transaction is completed. So for most purposes a cash transaction where I give you cash and you give me the goods is viewed as riskless (even though it's not as you point out).
But in your example, you don't need to invoke a security guard stealing your apples. There is a _ton_ of credit risk already just by the invocation of the electronic payment. Your funds will not hit the account of the merchant in question for _a long time_ (usually modeled as 1-3 days for risk purposes in the US). All kinds of things can happen in that time. The merchant has extended you credit and has taken on credit risk. They've done so because of the chain of credit and agreements that the banks, processors and networks have worked out.
If Mastercard gets a bunch of its transactions reversed because a Congressional panel gets huffy about pornography, that potentially blows up transactions for apples miles away. That's why this is mostly a conversation about risk management, not morals. To the apple vendor's bank, they largely don't care if you buy porn, they just don't want their settlements caught up in the blowback.
They could try cash-by-mail, like Mullvad.