I hadn't heard of it until your post. It's a very interesting concept. Similar freelance sites require the freelancer to write proposals, which can take up time that could otherwise be used to work. And not all gigs posted are bona fide.
What seems different about Amazon's idea is that the requester has to pay them up front, which may weed out some of the less serious players.
On the other hand, even by its very name, this seems geared for people willing to work for very small amounts of money. That may do for people in less developed countries or others who really need even small money. But it doesn't seem viable for US or other first-world workers, for the most part. Surprising, then, that Amazon is doing this.
Maybe workers can make money by doing the really repetitive tasks quickly.