Paypal tried making money on the float, but it didn't work. Amazon has a somewhat better chance of pulling that off since they don't have to ignite a fraud clusterfuck by giving out cash to get users like Paypal did. More importantly, their users have much less incentive to cashout since they know that the store of all stores will take it.
But the interest they could make on the float is nothing compared to the tithes they pay to the credit card companies, especially with the damn rewards cards. Just getting their prime customers to sign up for EFT, for normal Amazon purchases, has got to be worth a fuckton of money to them.