Amazon is the shipping company for a number of the products available for sale within other stores in the Pro Rebate Rewards. Amazon itself is not a store, it's a platform that hosts stores and some of these stores are on the app
So Amazon is involved in helping save on the shipping costs and quality of delivery ability for products within the app and any product that you can buy on Amazon you can easily find that same product within one of the other 4,500 stores on the app and get pro rewards.
Different stores will be available at different times depending on when they offer rewards so maybe Amazon will be one of the direct shopping options in the future.
Also remember the iPN mall is coming out soon and will have many of the same products as Amazon but priced better.