Lyft already had the ability to charge my credit card, as they do for almost everyone I assume. Robinhood actually just deducted the amount from my balance and then sent me a "you have negative cash in your account please add funds or we will sell positions to cover" message, which is a bit bonkers in itself.
Hmm I guess it's tricky with these two because they can get around in-app payment for their core business. Sneaky!
It's not fundamentally necessary, old-school predatory businesses like Columbia House were able to survive without an automatic way to collect on debts.