It's uniquely online processors that do not refund fees.
They had refunded fees but iirc it meant they were paying a small fee per refunded transaction (which in a sense subsidizes apps that incur frequent refunds) which they wanted to stop doing.
A quick search showed they started refunding fees in 2012 and stopped in 2017. Here's a discussion from the past couple years: https://news.ycombinator.com/item?id=22371330
It sorta makes sense, since the online processors are usually middlemen that only make money on the fees they themselves charge. I could see their perspective being "we provided the service; that service is not un-provided because the customer returned the item and you sent them a refund... in fact, you used us even more because we had to mediate the refund transaction!"
I think if the fees were just a few percent, I wouldn't have too much trouble accepting that logic, but from Apple, where it's 30%... not so much.
The card issuers don't care quite as much about keeping the fees in a refund scenario, I guess... though I believe you don't get everything back during a refund, regardless.
In fact, you will pay more on the scheme fees to process a refund, as you need to send another message into the scheme's network - often charged per message or per byte etc.
Interchange is not payment scheme revenue - it is just passed through to the issuing bank.
That is primarily an issue for cross-border credit or debit card transactions.
Scheme fees are much lower for domestic transactions as far as I'm aware, though.
Generally, Visa and Mastercard simply charge more per transaction for different cards in the US. For example, rewards cards cost merchants more, which is why many gas stations won't accept them.
Your bank account -> visa/mastercard/discover -> PayPal's merchant bank account -> the person you're paying bank account
PayPal only replaces the ccs when you pay with their wallet. Otherwise they're just orchestrating the money flow (because integrating with the CC companies is a gigantic PITA)
Stripe on the other hand is a credit card processor, and as a merchant you get a Merchant account, the same like Authorize.net who used to be the largest online processor before Stripe.
Back in the day (and I have been out of that market for over a decade) Authorize.net absolutely refunded processing fees on refunds
Auth.net is a gateway/portal that facilitates CC transactions, and while you may be refunded on processing fees for the use of the gateway, someone, somewhere is likely eating the interchange and most likely occurring at the merchant level.
TSYS, Vantiv, PaymentTech are processors. your merchant account is usually with (i think) the acquiring bank that's part of the Visa/MasterCard/Amex/Discover network.
Uniquely those that provide some higher level thing, like PayPal or Stripe. If you have a merchant account, you can get an actual refund for online purchases.
This is for AML, not because of fee refunds.