According to the U.S. regulations, protein shall not be declared on labels of products that, other than ingredients added solely for technological reasons, contain only individual amino acids. 21 CFR 101.36(b)(2)(i)
Products that are sold as amino acid supplements only should not declare protein on the label. Products that are sold as nutritional supplements that have a protein-containing base, like milk or soy base and added amino acids, would declare protein on the label. This section pertains to Supplement Facts labels (not Nutrition Facts).