...because Intel will pay them money, which can be exchanged for goods and services, for letting them do so?
I'd say either AMD doesn't have the production capacity for whomever the customer is (Apple?) or its something more nefarious like preserving ME.
Given that AMD is still a close second in graphics, maybe it really is a way to increase volumes for their GPUs. But that still suggests there's some reason they can't deliver the same with their own CPUs.
It's easier to see this from Intel's perspective: Intel's biggest threat is from the onrush of GPU-driven computing, and Nvidia is the market leader there. The classic play is to starve them of oxygen by leveraging existing channels to get them out of the gaming notebook market. Thus comes this weird saga with AMD and Raja which is a win win deal in fact: Intel gets ammo to fight Nvidia now and a key hire for their own development later, and AMD gets another source of cashflow and marketshare plus a graceful exit for what has been reported as shaky, conflicted executive management at RTG. Although they've delivered decent hardware and software recently, there have been numerous PR flubs from the group and the business unit performance is questionable overall. There is an open question of what happens to RTG later on, but perhaps the answer is simply to survive in Intel's shadow again.