Yes, they're loyal to whoever pays more and has more work. Slapping Amazon deliveries on top of Uber pool would mean more work and better unit economics, which should ideally translate into more money for the drivers.
The Amazon delivery approach would require them to drive from the city, to a remote fulfilment center to pick up goods, and then drive back to the city to deliver them. Is there a different way this can be done?
They could probably even use amazon lockers to optimize the process.