It's far from clear to me that that's true; soldiers, more or less by definition, are people who are more than usually willing to be killed in warfare. They are also typically engaged in (actual or attempted) killing of their own.

On the other hand, they commonly get killed in much greater numbers than civilians do, even when international law is being disregarded.

So the principle in question is really something like "If something doesn't merit killing a small number of innocent third parties, then it doesn't merit killing a large number of people who are actively trying to kill your people and who are willing to risk death in the process".

Of course, that doesn't make such a good soundbite. But I think the details that make it less of a soundbite have to be considered when evaluating it.