The problem with being anti-slaughter is turning horses from livestock into companion animals. I am one of the biggest horse lovers on the planet, but they are LIVESTOCK. I do not want anyone telling me my horses cannot stand in the pasture, rain, snow or shine. God designed them to do this. I do not want anyone telling me my horses cannot eat grass, or round bales, or drink out of a stock pond because the water in it is not clean. God designed them to do this. I do not have to clip them, groom them, bathe them, or comb the knots out of their manes. I do these things because I enjoy taking care of them, but I do not HAVE to do them.
The slaughter market gives horse prices a "bottom", and as you can see by Craigs list, etc, we do not have a bottom to the market, so we have a LOT of free horses.
Euthanized horses have to be buried, or hauled to a "composter", at great expense, and when you cannot afford to feed the horse alive, how can you pay $500+ to take care of it dead?
As evidenced by the current state of affairs, be careful what you wish for, you just might get it.