keyplus
We do earn, it's called invisible earnings which comes from such things as tourism, insurance etc
http://archive.treasury.gov.uk/pub/html/top/to p7/main.html
An example being, when I was an instructor at the School of Infantry, we taught minor battlefield tactics to junior officers from the British Army. Occasionally we had overseas students on courses from friendly countries such as Saudi Arabia, Malaya etc.
Having just one overseas student paid for the entire course, and more countries send their junior officers to be trained by the British Army than by the the Americans