I would think the usurious interest rates they charge would provide a sufficient income.
What they are doing is maximizing revenue by issuing cards to irresponsible consumers and then charging good customers exorbitant interest rates to make up for the losses. And they are also able to do this because there are too few banks and too little competition, thanks to lobbying to make sure the anti-trust regulators remain asleep.
Yes. They already get a 3% cut of every sale plus 20-40% interest on balances. Time to reign them in.