Years ago I worked for one of Amazon's shipping operators, whatever we wanna call them, and the rumor was that Amazon had already squeezed contracts to the point that on their own merit these contracts were not or only barely profitable, depending on the math. It was said to be still interesting to have the contracts, for scale and reputation. But then it may just not be sour grapes for FedEx, but rather that the numbers weren't compelling them to stay in a relationship likely to be dumped from anyway, sooner or later.

I bet amazon exploited the weaknesses in the contracts...

Delivery to San Fransisco - amazon delivers. Delivery to an island off Alaska - Fedex has to take it for a flat rate that nowhere near covers costs.