Presumably they'll want to deactivate the accounts for people who don't settle their bills, and they'll want to keep out folks who don't have accounts. This seems attackable to me, probably because they haven't released enough details about how it works.
How would you even define fraud in that context? Presumably there are things you could do to confuse or mislead the technology, but it seems to me that as presented the onus is on Amazon to just charge you the correct amount based on what you leave the store with.