Wouldn't loose any sleep over it.
AMZN contracts out airline ops so they don't have to manage it (and for whipsaw of course). Same with trains and container ships.
They contract out delivery drivers so they can keep the drivers at arms-length, but they still want to manage driver behavior for efficiency and branding reasons. Planes, trains, and ships already operate about as efficiently as they can.
Unlike planes, trains, and ships, delivery drivers can create public brand-image and liability problems with reckless driving.