"Labels on packaged food regulated by the Food and Drug Administration (FDA) must provide the statement of quantity in both metric terms (grams, kilograms, milliliters, liters) and U.S. Customary System terms (ounces, pounds, fluid ounces). For meat, poultry, and poultry products, which are regulated by the United States Department of Agriculture (USDA), the statement of quantity need only be expressed in U.S. Customary System terms. The use of metric measurement is voluntary because the Fair Packaging and Labeling Act (FPLA), which regulates labeling of consumer commodities, exempts meat and poultry products from metric statement requirements. (See 15 USC 1459(a)(1)."
Comments
I googled, and this was the answer, there is a separate similar requirement for metric labeling for cosmetics and the like.
https://www.packaginglaw.com/ask-an-attorney/are-both-imperi...
"Labels on packaged food regulated by the Food and Drug Administration (FDA) must provide the statement of quantity in both metric terms (grams, kilograms, milliliters, liters) and U.S. Customary System terms (ounces, pounds, fluid ounces). For meat, poultry, and poultry products, which are regulated by the United States Department of Agriculture (USDA), the statement of quantity need only be expressed in U.S. Customary System terms. The use of metric measurement is voluntary because the Fair Packaging and Labeling Act (FPLA), which regulates labeling of consumer commodities, exempts meat and poultry products from metric statement requirements. (See 15 USC 1459(a)(1)."