Non-partnered ltl carrier refusing to deliver to Amazon distribution center

We have a shipment that has twice now been picked up at our dock, but then was denied to deliver to Amazon by the two separate non-partnered ltl carriers that made the pick-ups, FED-Ex and R&L. Shipments were returned to us both times. Both carriers simply replied back "We do not service Amazon." We have never experienced this before.

What is going on?

Is Amazon refusing to allow certain carriers deliver to them, or are certain carriers just refusing to deliver to Amazon? I cant get a straight answer from the carrier customer support folks to see who's end the problem is on. Thanks much.