The second you approve of a policy that restricts action X based on moral grounds, you have defined a vulnerability that a less ethical enemy will exploit.
Furthermore, when you're in a war, it's chaos. Bad stuff happens. Collateral damage happens. You certainly don't plan to inflict 1000 civilian casualties, but you can predict that in a city of 1 million people undergoing an all out conflagration, there will statistically be civilians killed, displaced, wounded, orphaned, starving, etc. You don't stop a war just because you're better at math.
War also isn't the first choice of a rational society. Diplomacy, negotiations, sanctions, pressure, demonstrations, all these kinds of activities are intended to solve the problem before it degenerates into war. But there is always another side, and if it degenerates to war, it's because at least one side was acting in bad faith. ISIL isn't even acting as a rational society. They don't negotiate - they enter an area, kidnap and rape the girls and take them forcibly as wives, and kill, conscript, or indenture the males. They use civilians as human shields, betting that an opposing force won't bomb their headquarters if they have them located in a schoolhouse full of children.
An outside society can do two things: allow the continued expansion of slavery and genocide, or attempt to stop it. If non-military resolutions fail, what would you have them do? "Sorry, you can't fight those insurgents because they duct-tape kidnapped children to the front of their vehicles." "Right, we'll just let them continue on their homicidal path because we can't place those children at risk."
It's not like anyone in the West wants civilian casualties. The moral high ground may not be perfect, and it may not be absolutely 100% civilian casualty free, but you can't claim a millimeter of moral high ground if you let the atrocities continue unchecked.