Food for soldiers in the trenches during World War One was at times considered a luxury. Getting decent hot food from the field kitchens to the front line trenches could be impossible when a battle was either imminent or in full flow. When soldiers were at stand-down, food was easier to acquire.
I suppose you can see why the men complained if the food was not very good, for some it would have been their last meal. They may have taken the attitude that they were fighting for their country and their country was not feeding them properly.
My grandfather fought in the WW1 and he said that at all the field kitchens there seem to be plenty of Plum jam and not a lot of anything else. When he returned home after the war he never ever again touched Plum jam.