Because in your quiet assent to what was once considered butchery you know you're not alone. These days, much of the public raises no objections when our forces kill their non-combatants. After all, we've had plenty of time to get used to it. For instance, estimates of deaths for which Genghis Khan and his men were responsible in the thirteenth century run as high as 40 million, with only a fraction fighting men.
But, with the onset of the Age of Enlightenment in the West, the idea of confining warfare to the battlefield was, if not adhered to, at least given lip service.
Until the advent of aerial bombing.
The indiscriminate killing arising from a bombardier's difficulty distinguishing between military and civilians demanded a cover story. Calling bombing, in conjunction with shelling, "total war" legitimized it and ensured that the military could continue to cast a wide net when it came to victims.
Here's Wikipedia on RAF Marshal Arthur "Bomber" Harris: ". . . in a conflict where attacks on civilian targets had not only been initiated by the enemy but considered a largely acceptable aspect of 'total war'. . . Harris' strategy of carpet-bombing German cities was coherent and certainly dealt great damage to the Axis heartland." I'm sure that, had they known, those who died in the Dresden, Hamburg, and Tokyo fire-bombings would have drawn consolation from going up in a blaze of "coherence."
But what about enemy initiation of attacks on civilian targets? Use that as a justification to respond in kind if you want. Just remember that any righteousness equity a nation derived from its status as a victim is squandered when it retaliates many times over.
Another argument for killing civilians in WWII was that their industrial incorporation into the war effort made them as culpable as combatants. In fact, this was just another lame excuse to demoralize a country's citizens by laying waste to large swaths of them. But, unless civilians are annihilated on the scale of a firestorm or a nuclear attack -- almost nobody left to demoralize, in other words -- bombing only serves to harden their resolve.
Meanwhile, the American public, still getting a lot of mileage from the American Revolution, subconsciously blames the Iraqi public, for example, because it lacked the gumption to overthrow a tyrant. Like we would have if, instead of just taxing to the max, George III had mass-murdered like Saddam Hussein.
It was under the guise of total war that we dropped the atom bomb on Hiroshima and Nagasaki to kill Japan dead and put the fear of God into Russia. But we also wanted to find out if the thing worked in the field. In the process, we not only reduced Japanese civilians to the level of experimental animals, but subjected them to deaths more agonizing than any lab rats.
Once killing the enemy's civilians became realpolitik, the average American citizen would as soon jeopardize his credit rating as risk devaluing his credibility by going against the current of the consensus.
On a parallel track, most of us, including much of the working poor, don't think anti-labor policies apply to us. Between owning a house of inflated value or pinning our hopes on the lottery, watch who you're calling "labor," buddy. Likewise, despite 9/11, we fail to acknowledge that if civilians elsewhere are fair game, we are as well.
Why is it Americans don't fear reciprocity for inflicting casualties on the civilians of other nations? Actually, since 9/11, we do. But, hey, we're Americans and you can't talk to us that way. Besides, despite a monumental terrorist attack on our soil, we still experience ourselves as invincible.
Worse, when we claim our military bears scant responsibility for civilian deaths, we reek of sanctimony. After Hiroshima, we were forced to wrap ourselves in denial to ward off the realization that America operates in, uh, broad strokes.
To be fair, "exceptionalism," as Noam Chomsky wrote, "seems to be close to universal. I suspect if we had records from Genghis Khan, we might find the same thing." In fact, Mongolia, apparently suffering a severe role-model deficiency thanks, no doubt, to their years as a communist state, has recently made the 40-million man a national hero.
When we accept targeting civilians, the American public signs a contract that it's fair game as well. In effect, we're all transformed into warriors. However brave and selfless that strikes us, our courage at this point is vestigial, just a legend based on the exploits of soldiers from wars long ago.