I think the idea is "not Uber". If Uber is the only payment processor, then they look more like an employer and less like the middleman they want to claim to be. The money is coming from Uber to the driver rather than from the driver to Uber as their commission.
That is normal, e.g. if you travel through a federated regional bus/train transport system, you're paying through the platform and the company is processing your payment, sending money to individual transport operators. See flixbus as the most extreme example, but there are tons of these across Europe, many of them formed during post-communist privatization. Its similar with parcel delivery companies, these even are forced to have the PPL brand on their own cars!
