9TO5Google quoted Jeff Bezos when he attributed unfavourable terms as being the reason why they stopped selling the Chromecast online. However, my thinking is more in line with the commentors here in calling this one out as BS.
As the cast device doesn't have a store and it would seem unlikely that Google would charge app developers a commission to add a cast feature. There is nothing it would seem preventing Amazon from supporting its services on the device other than an absence of will.
It is more likely that the 'unfavourable' terms Amazon wanted here were for Google to give it a bigger cut of the sales revenue in exchange for it to continue selling a competing product to its Fire stick in the storefront.