I don't pretend to know Amazon's view of this particular product, but I will observe that Amazon's business model involves making huge investments to become everything to every consumer. Remember how one time they were just a purveyor of mail-order books? Then books and movies? Now, everything, to the point where they have big-box stores shaking in their boots.
There are plenty of market segments where Amazon is investing big-time in becoming the market leader, even at the expense of short-run profits. I don't know what synergies they see between the payments business and the retailing business, but I know this, Jeff Bezos is a hell of a lot richer than I am, and he didn't get that way (at least so far) except by having more products work out well than badly.