Really? Where's the "private gain" for some 18 or 19 yr old kid who is sent (ordered) to some hell hole half way around the world to get shot at or blown up for buttons money?
These kids weren't ordered to join. They did so of their own free will knowing full well what they were in for, which for most of them is to play real life Call of Duty (warning: may be traumatizing for some) with real people and real life guns to the soundtrack of The Roof is on Fire. Getting paid for that is a bonus. It's sure a hell lot more than what they would make at McDonald's per hour. Also they'd get called heroes back home.