Originally Posted by
krazykanuck
...some form of revenue to United for spend on their co-branded cards while spend on the CSR, there wouldn't be any ....
Chase buys miles from UA. If you use a MileagePlus co-branded card the rewards are automatically deposited in your UA account, and Chase pays for them. If you use a card that earns Ultimate Rewards, United isn't involved unless you convert points to UA miles, at which point Chase pays.
The price per mile may not be the same in these two contracts, but the big difference is engagement with MileagePlus. The customer earning UR isn't committed to UA.