How have banks been unethical in encouraging irresponsible borrowing any more than Hershey's has been unethical in encouraging irresponsible overeating? If banks offer me appealing rates that I may use to swamp myself in debt or Hershey's offers me cheap chocolate on which I may gorge myself into obesity, they are providing me a useful products that I may use to my benefit if I'm responsible. Hardly unethical.

Banks buy miles and are selling them for a higher price than they pay for them. Sure - clever customers might be getting them for a much lower price than their peers, but the banks are happy to keep buying and selling them because on the whole they turn a profit doing so. It's no different than a retail store that is happy to lose money selling teaser items to frugal customers because it also leads some of these same people as well as others to splurge on their high margin goods.

If anyone became a victim (banks of their customers alike), there is an easy out... stop doing business involving miles.