Soldier of Fortune magazine, which is still published to this day, is rather infamous for it's glorification of foreign wars, as well as it's use as a platform for mercenary recruitment. During the seventies SoF ran piles of articles on the war in Rhodesia, which obviously proved attractive to a fair amount of Americans. The war was cast as a white, capitalist society fighting against communist, soviet-backed, black african rebels, which tied in well with the anti-communist sentiment many Americans had (and still have).

Ward Churchill, who has been somewhat disgraced because of an essay dealing with 9/11, wrote a good article on this topic which dealt with how Americans were recruited.

Very few Americans actually managed to join the Selous Scouts (though many people probably brag about it), the Selous Scouts were one of the most infamously tough units in the world, with training that was more brutal than US Navy SEAL training currently is.