You aren't thinking meta enough. What we "charge" our allies is effectively the same thing that happens to welfare recipients: they become dependent upon us, and therefore we can dictate their actions.
Our allies don't have to "waste" money on defense, we will take care of that for them. Their concomitant lack of militart might also means our allies don't have the power to effect (or significantly affect) international policy like we do.
This really isn't all that surprising. To reinforce this regime, we outspend on defense by an order of magnitude more than any of our allies, just to ensure they are completely demoralized should they have any inclination of getting uppity and trying to build up enough martial power to be able to throw their weight around.
Hell, the UK couldn't even mount an air campaign against Gaddafi without our logistical support, and the man wasn't even fighting back. Imagine what would happen if we didn't want the UK to interfere in Libya?
To top it all off we inject the hegemony of the petrodollar, thereby propping up our currency in the international realm and ensuring it becomes the de facto reserve currency of the world.
We're really quite adept at ruling the world. *That* is what we get, and *that* is why we don't charge them in currency—we just take their souls instead.