It’s a principle in warfare, and particularly warfare since WWI, that whatever you do in war, can be done TO you with no repercussions. It is why the US has a standing stated policy that they will nuke anyone using an ABC (atomic, biological, chemical) weapon. If you attack with a weapon of mass destruction the reserves the right to nuke you.
Same principle. If you attack civilians you just authorized attacks on YOUR civilians . If you attack non-military targets you just authorized attacks on your non-military targets.
All that said, any airport is a military target in time of war.