Civilians can leave a city. They are not forced to remain, unless they are slaves.
And cities are legal military targets. The British, and to some degree, the Americans bombed cities in WWII with the express view of killing war workers, it being seen that a war worker was just as valuable to the war effort as a soldier on the front line.
But the Germans could and did evacuate all non-essential persons. Your statement is simply not correct.
Walt