All these corporate starting own currency makes no sense for decentralization but it may be useful for the companies to reward platform users and keep track of several things on blockchain. I wouldn't invest in any of these to be fair.
These company will definitely try to modify the blockchain system as they sees fit, there should actually be taxonomy or categorization for cryptocurrency since I expect there gonna be a lot more variation that modifies the blockchain system.
However, amazon creating their own crypto is the same as those private company creating stablecoin, won't change anything if any it could let crypto users purchase through their platform.