The reason why sheep lung is banned in the US is a result of food safety concerns.
Stomach fluid can enter the sheeps lungs during slaughter and increase the risk of foodborne illness from pathogens and bacteria.
So due to food safety concerns, the U.S. Department of Agriculture or USDA enacted the ban on all animal lungs including sheep lungs for human consumption in 1971.
The USDA established a zero tolerance policy for such contamination to prevent any disease that could be spread and ensure protection of the consumer.
People can still slaughter their own sheep and eat the lungs but the sheep lung cannot be sold for human consumption in the United States.