pbi nailed it, really. We have to be able to credibly hold ourselves to a higher standard, and the law is there to ensure that those who do not are held accountable. It was noted in the sentencing that no precedence could be found amongst our allies for this sort of situation, and it certainly isn't one that has an easy, unambiguous, universally satisfying answer. It is clear the Semrau acted without malice, and I would hypothesize with a sense of great compassion, and that he realized that he had make a difficult decision. The trial transcript, as I recall, stated that he ordered the rest of his team out of the area saying something to the effect of "you shouldn't have to see this". He accepted responsibility to some degree, though he did not make any effort to explain himself. I don't begrudge him that, it is his right, and was probably a legally prudent decision.
I suspect that the discussions about this case will go on for a long time, and will definitely be a case study in military ethics and the application of the Laws of Armed Conflict. It strikes me that a reasonable balance was struck in that while the convicted's career in the military is over, his life goes on, he can return to his family, and he can carry on. I suspect that should he seek employment there will be many sympathetic to his circumstances who will seek to help him. I don't see why he might not apply again, the sentence has left that door open.