The reason you can't see people drinking beer in beer commercials is not due to a law or government regulation, but rather due to industry self-regulation and television network policies. Networks generally discourage showing alcohol consumption on camera to avoid potential backlash or restrictions, and beer companies voluntarily follow these guidelines to ensure their advertisements are widely accepted and aired on major networks. This self-imposed rule helps prevent the advertisements from being banned or limited, maximizing audience reach. Thus, while it's legal to drink in beer commercials, it's rarely done to comply with network standards and collective industry practices.
