In the case of Chicago et al. v. Fieldcrest Dairies, Inc., 1941, the U.S Supreme Court ruled in favor of Fieldcrest Dairies, a milk distributor that had been charged with violating an ordinance by selling milk below minimum prices set by the city of Chicago. The court held that this price-fixing ordinance was unconstitutional as it violated due process rights under the Fourteenth Amendment and exceeded police power limits because it did not serve public welfare or safety interests. Furthermore, it interfered with interstate commerce since some milk sold came from outside Illinois state lines. This ruling affirmed that local governments cannot interfere with free market competition through arbitrary price controls unless there is a clear connection to public health or safety concerns.

In the dissenting opinion for Chicago et al. v. Fieldcrest Dairies, Inc., it was argued that the majority's decision to uphold a tax on out-of-state milk dealers violated principles of interstate commerce and equal protection under law. The dissenting justices believed that this tax unfairly burdened non-resident businesses by imposing higher costs than those faced by local competitors, thus creating an unfair trade barrier between states. They also pointed out inconsistencies in how the tax was applied, with some non-residents being taxed more heavily than others based on arbitrary factors such as their business structure or location of operation within Illinois state lines. This selective taxation was seen as discriminatory and contrary to constitutional guarantees of fair treatment under law.