One can understand that they can aggressively work the rules and sell cheap point-to-point tickets (and/or have access to consolidator-priced trips).

But I'd still wonder where they find any edge that lets them "discount" OWE tickets.

Afaik commissions paid by the airlines have gotten to be almost a joke, and credit-card companies aren't known for giving stuff away - you're paying for those miles and rebates one way or another.