Cavel International -- a Belgian company -- had been slaughtering 60,000 horses a year and exporting the meat to places where people like to eat horse meat: Belgium, France, and Russia. It was the last horse slaughtering plant in the country.
Here's Posner's opinion for the 7th Circuit (PDF):
Horse meat was until recently an accepted part of the American diet--the Harvard Faculty Club served horse-meat steaks until the 1970s. No longer is horse meat eaten by Americans..., though it is eaten by people in a number of other countries, including countries in Europe; in some countries it is a delicacy. Meat from American horses is especially prized because our ample grazing land enables them to eat natural grasses, which enhances the flavor of their meat.The question was whether the law violated the negative Commerce Clause or the Due Process clause, and both issues depended on whether the law was wholly irrational. Posner serves up a meaty opinion.
[Cavel argues:] The horses will be killed anyway when they are too old to be useful and what difference does it make whether they are eaten by people or by cats and dogs? But the horse meat used in pet food is produced by rendering plants from carcasses rather than by the slaughter of horses, and the difference bears on the effect of the Illinois statute. Cavel pays for horses; rendering plants do not. If your horse dies, or if you have it euthanized, you must pay to have it hauled to the rendering plant, and you must also pay to have it euthanized if it didn't just die on you. So when your horse is no longer useful to you, you have a choice between selling it for slaughter and either keeping it until it dies or having it killed. The option of selling the animal for slaughter is thus financially more advantageous to the owner, and this makes it likely that many horses (remember that Cavel slaughters between 40,000 and 60,000 a year) die sooner than they otherwise would because they can be killed for their meat. States have a legitimate interest in prolonging the lives of animals that their population happens to like....Go to the PDF if you want to see how Posner manages to work in a photograph of a lion eating a birthday cake made out of horse meat. John Stuart Mill... a photograph of a lion eating a birthday cake made out of horse meat... I love these erudite opinions!
Of course Illinois could do much more for horses than it does--could establish old-age pastures for them, so that they would never be killed (except by a stray cougar), or provide them with free veterinary care. But it is permitted to balance its interest in horses' welfare against the other interests of its (human) population; and it is also permitted to take one step at a time on a road toward the humane treatment of our fellow animals....
But even if no horses live longer as a result of the new law, a state is permitted, within reason, to express disgust at what people do with the dead, whether dead human beings or dead animals. There would be an uproar if restaurants in Chicago started serving cat and dog steaks, even though millions of stray cats and dogs are euthanized in animal shelters. A follower of John Stuart Mill would disapprove of a law that restricted the activities of other people (in this case not only Cavel's owners and employees but also its foreign consumers) on the basis merely of distaste, but American governments are not constrained by Mill's doctrine.