Drugs are illegal. People sell drugs. If there were no market, there would be no drugs. Horses will be stolen if there is a market. The plant in Alabama routinely processed stolen horses.
Back in the old west days, they would have have public hangings for horse thiefs. It happened quickly and without appeal. Since public hanging is unlikely to take place, what do the pro-slaughter people propose to do about theft? I put years of my life, a lot of my soul, and a whole lot of money into my horses. I don't want them stolen and killed for profit.