Definition 1 of 1
The expression regularly used in news reporting stands for NATO and its allies including Afghan and Pakistani forces firing and killing each other by mistake!
You probably haven't heard of friendly fires in other theaters of war as much as in the Afghan. You know what a friendly cricket match is, a friendly boxing match is, but how would you ever explain what was the friendliness in the act of someone firing at you, a friend, and most certainly killing you?
Didn't the reporters and news agencies think for a second and find another more apt expression? It's the way of that imperial language probably, like during the East India Company times the 'pears soap was made and sold to the brown man but only to clean the white man's burden'
An expression as simple as 'mistaken fire' would have been much humaner than 'friendly fire'.