Two pet stores and a Missouri dog breeder attacked the constitutionality of Chicago’s “puppy mill” ordinance based in part on the dormant commerce clause.The city council concluded that dogs bred in deplorable large-scale commercial operations are abandoned in large numbers because they are often afflicted by health and behavioral problems.To combat this problem, the puppy mill ordinance says pet stores in Chicago can only sell dogs that are obtained from animal shelters. The remaining option for …