with absolutely no return on investment for them.

They show ads unless you pay for the super plus version of Prime. So by streaming they get ad revenue from the companies advertising even if you don’t watch the ads. The companies pay for the ads by increasing the price of what they sell- and they don’t sell only on Amazon but everywhere.

So when you buy a bag of chips at the local store, some of that money is going to Amazon because you had their streaming on.