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.
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.
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.