Why is proof not required to be stated on bourbon labels by US law?

The correct answer is based on the understanding of how alcohol content is presented on labels. In the United States, bourbon labels are required to include the alcohol by volume (ABV) percentage, which indicates the strength of the spirit. Proof is a measurement derived from the ABV; specifically, proof is calculated as twice the ABV. Therefore, if a consumer is aware of the ABV percentage, they can easily determine the proof without it needing to be explicitly stated on the label.

By using ABV as the standard, regulations allow consumers to infer the proof content themselves. This approach streamlines labeling requirements while still providing the necessary information regarding the product's alcohol strength. Thus, there isn’t a legal requirement for proof to be separately labeled, as it is redundant when the ABV is already disclosed. This system also facilitates consistency across different types of alcoholic beverages, where ABV is typically the primary measure used.