Find some perspective.....
1) They were "asked" to do exactly what they came in for. They were "asked" to do their jobs. Nothing has been too much. Most WWII Vets went to war and didn't come back unless they were wounded or the war was over. And seeing as how most veterans of the "War on Terrorism" enlisted after 9/11, combat should not be the last thing they expected when they signed. Especially a soldier or a Marine.
2) Most Post Traumatic Stress is extremely temporary and is mostly grounded in a sense that nothing matters. The extreme actions of combat can leave a "soldier" with a sense that holding a job in the US is simply boring or unimportant. Even something as simple as comparing combat action and your kids report card is a hard thing to balance. Most troops find their way through this on their own or with local support on the base.
3) The fact is that most of these suicides are a result of coming home to find their world wrecked by those they trusted most.
There is also another less talked about side to this PTSD. Adrenaline junkies have a hard time settling down and seek dangerous things to pre-occupy himself. This isn't based in depression, but thrill seeking to re-capture that feeling of invulnerability and rush. There is a reason motorcycle accidents have been on the rise.
Tacking on "PTSD" everytime a soldier gets sad, or suffers a paper cut, or commits suicide, trivializes those who truly suffer from this disorder.