From an attorney on another site who works on animal rights issues. I can word it no better, so I'll post his thoughts:
I don't really see what the objection would be. You have these horses, they are unwanted, wasting away, and presumably suffering, so you have two choices:
1. Euthanize them, as you would a pet.
2. Slaughter them (humanely, etc.), and utilize the remains for some socially productive purpose.
So pick one.
You will notice the absence of the usual third option: place them in a horse rescue organization, either for re-homing or to live out their days in a pastoral setting. I don't view this as a viable large-scale solution for horses any more than I do for the more traditional companion animals. On a small scale, to the extent that dedicated groups and volunteers are able to provide the necessary facilities and care, go for it. Farm sanctuary places are living reminders that consumption comes at a cost. Sometimes seeing first-hand the lives that were spared makes us just a bit more mindful of how we consume the products derived from the lives that were not.
I don't foresee horses becoming a food source in this country, ever. There is just too much psychological baggage attached to horses for that to happen. That said, most of my belts and all of my gunleather is horsehide...and I sleep OK at night.
In my animal law capacity, I have far less of a beef (no pun intended) with slaughter and consumption than I do with it in the factory farming/mass-scale slaughter context. The laws in my state are actually pretty good regarding companion animal abuse, neglect, and proper care, and we're seeing improvements in enforcement. What is lacking tends to be on the federal Animal Welfare Act side of the coin. But that's a different discussion altogether.
What I try to have is a level of compassion for the dignity of living beings of all sorts to live and die in a decent fashion. Starving in a guy's field, because he ran out of money to feed you, is not a decent life or death for a horse. Being abused or exploited is not a decent life. Being confined to an unnatural and unduly small space is not a decent life. Being used to test cosmetics or household chemicals or some other crap we know full damned well shouldn't be ingested or put in one's eyes is not a decent life.
Contrast that with a deer that spends its days in the woods and fields until, early one November morning, a shot rings out and it drops to the ground, dead, soon to become the focus of many family meals for a reverent hunter and his/her family...there is dignity in that life. Ditto for the trout that gets caught in the afternoon and cooked over a campfire as the sun sets. Even livestock, cared for properly and given adequate space to do the things that come natural to them, can experience a life of dignity.
Obviously, I don't fit in well with the majority of the PETA/AnimalRights crowd. I think it is a question of minimizing harm and maximizing respect, rather than the plants-only perspective that so many others advance. The sad part is that there a lot of people like me, yet most are afraid to open their mouths around others who take animal welfare seriously for fear of the most petty sort of retribution, so our position goes largely unheard within our community and in the broader society.
Anyway, that's my 2c.