Though I don't agree with horse slaughter, It's kind of hypicritical because we slaughter cows, and chickens for consumption so really whats the difference, even though I will not be eating it. And as far as I know it is legal to slaughter and eat your own horses if you want. So while I understand that horses are more of pets, because we ride them and what not, other people only see them at meat, and money. It's a difference of culture.
It's the nature of the beast, if we didn't have so many back yard, irresponsible breeders just like in the dog world we wouldn't have so many unwanted animals.