The original premise of frequent flyer programs was to use seats that would otherwise be unsold to reward loyalty. As the USA airline industry has reduced capacity while simulataneously offering miles through partners the demand for award flights has exceeded the supply of empty seats. I think you are seeing an attempt to rebalance supply and demand by offering alternate awards.
It certainly appears that Chase and some other issuers have been pressing their affinity card partners (e.g. Continental, United, Marriott) to restrict earning through other cards. This doesn't really affect the supply of miles or points, it just forces the participants to use a specific card.