All transactions get propagated through the entire network of bitcoin users, with miners eventually also hearing them. So if you suggest that a mining cartel can somehow keep transactions from being broadcast, and keep other competing miners from hearing about them and mining them too, you are mistaken.
You are incorrect. If Amazon offers a downloadable client (or even one that runs from their website), which sends the transactions to their server, they have no obligation to forward the transactions to other miners.
So, if you are using an Amazon wallet app, and I am using some other wallet app, and you try to send me coins, how will I know whether you sent them if you only send them to Amazon's servers? It would essentially cut everyone using Amazon clients off from the rest of the bitcoin network. Why would anyone want to use such an app? Bitcoin transactions primarily work because they are propagated P2P through the network from person to person. Miners just sit on the perifere catching these transactions as they pass by and adding them to blocks.
You are conflating the publication of block solutions with the propagation of transactions before the fact.
You don't understand well the way Bitcoin works.