Ah HA! Okay, apparently it was made illegal because people's horses were being stolen and butchered. I can believe that, because someone in my family was constantly having his horses and goats stolen. He even found one of his miniature horses with its throat slashed, but I guess the people trying to steal it left in a hurry or something so didn't get to take it with them.
OF course that doesn't really explain why eating horse meat is illegal, when they could import the meat from a real butcher.