It is not really an innovation. This is basically private money that can only be used at a particular store.
At the dawn of industrial revolution large mills used to pay their workers with "scrip" -- a private type of cash that could only be used in company-owned stores. The company basically double-dipped: first profiting from the labor of the workers, then profiting from the mark-up on the goods and services "sold" to the workers.
Once you put money in Amazon cash, it goes to Amazon and nobody else. It is no longer "cash" or legal tender if you can't use it wherever you want.