Interesting question indeed, especially when you consider a few things, eg. we have Americans serving in our forces, although not many. Far more Canadians have served, and continue to serve in the US military, many of whom are MicMaq' or other indiginous peoples. Many Canadians also serve in Her Majesty's forces. A lot of these folks join foreign services, especially the Air Forces because Canada gives preference to fancophones when it comes to promotions, (our dirty little secret which is well known but not well advertised) and anglophones feel slighted, rightly so.
But you can also think of people like Mohandas Karamchand (Mahatma) Gandhi who proudly served with the British forces in many parts of Africa. So why not have Afghans serve in the Canadian, American, British, etc. forces, especially in their own country?