They were at war, there were men on the ground who would likely die if they didn't fly. The consequences resulting from them not flying because they hadn't had any sleep in the last two days were a bit more important than an airline pilot not agreeing to fly extra because their company was too cheap to have enough reserves to cover the sick calls.
If a military pilot in wartime doesn't fly then people die. If an airline pilot doesn't fly then United doesn't make as much money as they otherwise would, the Smith family is a few hours late to the beach, and some traveling salesman has to spend a couple of hours more spending his expense account at the airport bar. The two situations aren't even remotely comparable.
This was covered pretty well in the movie 12 O’Clock High, it was about missions, not hours flying. The hours those pilots flew during W.W. II were grueling, long and tough. Weather in Europe was always a factor and when the weather was clear they flew all they could before the weather was again a factor.