> So, why would anyone participate in this program, if the best possible outcome is that Amazon uses your idea, makes millions, and does not pay you a cent?

This is what happened to the author of Forrest Gump. The reason there was no sequel despite that movie's massive success was that he was too proud to sell the rights for the sequel when he'd gotten screwed on the first one.

However, it seems likely, given the first movie's massive success, that he could have gotten a pretty good price the second time around.

Always be careful with what you think the best case is.