It's cheaper to develop the app that authorizes you to charge and deducts from a balance ($10, for example) than the credit card transaction fees that are unsustainable at charges between $0.50 and $10. This is no different than how tollways charge your credit card on file to replenish a balance, and then deduct from that balance (or even how Twilio does billing).

TLDR Credit card fees are expensive, especially when your margins are thin and you are not yet at scale where you have pricing power to pressure processors to lower your CC fees.