In Orwell's Animal Farm, animals were told that extra work was voluntary, yet those who didn't volunteer got their rations reduced.
The whole point of George Orwell’s best known works, Animal Farm and 1984, is their depiction of dystopian, totalitarian societies in which language becomes just another instrument in the dictators’ arsenal through which to ensure control over the captive population. A metaphor for the manner in which Vladimir Lenin and Joseph Stalin subverted legitimate revolution for the purpose of installing autocratic political systems, the society depicted in Animal Farm exhibits the very same tendencies as those witnessed in the Soviet Union during the 1920s and 1930s. In the beginning of Chapter VI, Orwell establishes a scene in which, consistent with the surrealistic manner in which Soviet communism and Nazi Germany both operated, the government has convinced the people over whom it rules that the brutality to which they are routinely subjected is for their own good. It is worth recalling that the sign above the entry to the German concentration camp near Auschwitz, Poland, where over one million Jews were killed, said “Work Makes Free.” History’s most notorious death camp boasted at its entrance that working hard was the key to survival and freedom. It is in this vein that one interprets Orwell’s description of life on the farm. As he opens Chapter VI, he wrote the following:
“All that year the animals worked like slaves. But they were happy in their work; they grudged no effort or sacrifice, well aware that everything that they did was for the benefit of themselves and those of their kind who would come after them, and not for a pack of idle, thieving human beings.”
The animal cannot know it, or, maybe they should but won’t acknowledge the realities of their situation, that they have, in fact, become slaves. It is in this context that Orwell writes the passage specified in the “question,”:
“Throughout the spring and summer they worked a sixty-hour week, and in August Napoleon announced that there would be work on Sunday afternoons as well. This work was strictly voluntary, but any animal who absented himself from it would have his rations reduced by half. Even so, it was found necessary to leave certain tasks undone. The harvest was a little less successful than in the previous year, and two fields which should have been sown with roots in the early summer were not sown because the ploughing had not been completed early enough. It was possible to foresee that the coming winter would be a hard one.”
As noted, one of the defining characteristics of totalitarianism is the extent to which language is perverted for political means. Orwell’s 1984 is well-known for its examples of such uses of language. In Animal Farm, the same principles apply. The word “voluntary” is a manipulation of language for the purpose of convincing people to do what the government wants them to do, and is actually making them do. None of the animals has a choice. If they don’t want to starve to death, they will “volunteer.”