Most payment companies aren't able to (due to regulations) do anything particularly high-yield with funds they steward for their merchants. The amount of float a company like Amazon is able to make may factor into their revenue model, but I imagine they hold on to funds more for regulatory and risk mitigation reasons.
thanks for the links. I can't speculate at what their requirements are for affiliate payments. I only know the requirements Balanced is placed under as a PSP. I imagine Amazon must follow similar guidelines.