There is a lot of evidence to suggest that horse slaughter is a demand-based business, rather than a business that serves simply to dispose of excess horses from the horse industry (Sensible debate over unwanted horses would help anti-slaughter lobby and A Study of Equine Slaughter/ Abuse Patterns Following Closure of Horse Slaughter Plants in US). The demand for American horses principally comes from parts of Europe and Asia.
Horse meat is not consumed in the United States at any significant level. This is mostly cultural, but it's sale for human consumption is banned in California. However, many Americans do not know what horsemeat looks like, and if it were erroneously labelled, might purchase it instead of the beef or lamb they intended.
Horsemeat is a red meat with a purplish cast. Beef is a more scarlet red; lamb is a more pink red. Bison is a darker red. Horsemeat tastes sweeter than beef. It will not take seasonings the same way beef does but when highly seasoned, it can be mistaken for lamb. (Marinate a beef roast in a sweet bourbon to learn the taste of horsemeat.) Locally labelled purplish meat has been reported in even national chain grocers.
There is very limited demand for horse meat in Canada. That demand considers horse meat a healthy alternative to other meats.
In Mexico horse meat is consumed as a cheaper substitute for other meats.
In Europe horse meat is, like Canada, considered a healthy alternative to other meats.
Live horses are shipped to Japan as it is important to eat the horse meat within a few days of the kill.
All horses that were not bred for slaughter, which are the overwhelming majority from the US (there are unconfirmed reports of horses bred for slaughter in the mid west), have been administered drugs. Depending on when those drugs were most recently administered, they could render their meat unfit for human consumption. The EU recently initiated a plan to ensure the meat from the horses they slaughter is fit for human consumption: Microchipping to be compulsory for all EU horses
It would be good to add discussions that illustrate genuine buyers being shut out by kill buyers when wanting to buy horses.