Sun Tzu wrote that one feeds his army on the expense of the enemy. It's a bad practice to take away the food for the army from our own civilians, it has to be acquired from the enemy's subjects. And after the conquest the same, the population of the conquered lands have to feed the army.
This is age old axiom, it was practiced all over the world by people in places and times who never even heard of Sun Tzu. It is perfectly normal what the Germans did as far as the norms of a war goes.
Wars on large scale fought before, but the enormity of the two world wars was an entirely new challenge for which there were no proven solutions, but the age old axiom was still true.
In WWI there were little conquered lands which could have been used to live off, resulted in food shortages in Germany. Now they could reach for the resources of others, and they did, this is practicality.
The same practicality led to the corrections of the rations of the foreign workers.