In the 1942 case Natural Milk Producers Association et al. v. City and County of San Francisco et al., the U.S Supreme Court upheld a San Francisco ordinance that prohibited the sale of milk not produced within specific districts in California, despite challenges from out-of-state dairy farmers who claimed it violated their rights under the Commerce Clause. The court ruled that while interstate commerce is generally protected by federal law, states have certain powers to regulate local aspects of commerce for public health and safety reasons - in this case, ensuring access to fresh milk for city residents. This decision reinforced principles established earlier in cases like Baldwin v G.A.F Seelig Inc., where state laws were allowed to control economic activities if they served a legitimate public interest.

In the dissenting opinion for the case Natural Milk Producers Association et al. v. City and County of San Francisco et al., Justice Roberts argued that the ordinance in question, which prohibited sale of milk not produced within certain geographical boundaries, was unconstitutional as it violated both interstate commerce laws and equal protection rights under Fourteenth Amendment. He contended that such a regulation did not serve any legitimate public health or safety purpose but rather served to protect local dairy farmers from out-of-state competition, thus constituting economic protectionism. Furthermore, he asserted that this law unfairly discriminated against non-local producers by denying them access to markets solely based on their geographic location without considering other relevant factors such as quality or safety standards of their products.