It's not 30%, but a very large portion of VISA's profits comes from the fees they charge; VISA's fees are not capped to their "costs" of operating. VISA exists as a literal precedent that you can profit off a commission for a financial transaction.

The issue in question isn't whether Apple should be allowed to profit, it's whether they should be allowed to abuse their effective monopoly position to set their commission a literal order of magnitude higher than any other processor.