Most modern rechargeable batteries have fairly sophisticated chemistry and thus charge controllers needed -- you don't just dump electricity into them blindly and then take it out. It's not quite rocket surgery, but there's a reasonable level of engineering involved, especially if you want optimal performance with the newer standards (which increase voltage on devices which support it, etc.)
Often times that "reasonable level of engineering" only needs to be slapping a ten cent IC on a board with some manufacturer default application circuit.
Ah, I read "power bank", as that's what ikeyboy mentioned above. And when you search (google) "power bank" the results are all about portable batteries for topping up phones and whatnot. Does Amazon even brand/re-sell power supplies like jschwartzi is talking about?
I do hear chargers with a large number of ports called "power banks" - I think the implication is that it's like a line of ATMs serving many customers.