These were privately bought animals who were raised by private individuals and the government took half.
Don't confuse what happened in the US with what happened in the UK.
The government told you what you could grow, how much you had to grow, what you could keep for personal use and what you had to sell and at what price.
There is a pretty good documentary called Wartime Farm that shows the iron grip the government had on the food supply.
Because it was sold to the people as "equality" they decided to continue it after the war and rationing ended only in 1956.
Some government controls such as what kind of cheeses you could make did not end until the 1990s.
Pig Clubs - Growers signed up voluntarily. They got govt grain rations for the pigs (not all of which was used for the pigs, btw). The govt then -Purchased- half of the pig (at a market-rate price). The pig money reimbursed the pig growers for their garden scraps and labor. There was nothing compulsive about Pig Clubs.
“When they were ready for slaughter, which was a big day in the life of the Club, half of the carcasses were sold to the Government, to help with the rationing, and the remainder was divided between Club members, as either pork or bacon. “
http://www.bbc.co.uk/history/ww2peopleswar/stories/89/a4464489.shtml
The US government continues to tell farmers what they can and can not grow through allotments.