And to add to everything TxHorseMom said, which I totally agree with, what do you think we're supposed to do with the bodies after they've been "humanely euthanized?" Of course the bodies don't just stay there, they rot. Rotting flesh pollutes the groudwater bad enough as it is, which of course comes right back and infects animals and people with potentially life-threatening diseases. It's even worse if they have been killed with a "humane" life-ending chemical. This also leaks into the groundwater, and if it killed the horse, guess what? It's not exactly beneficial to those drinking the water. If horses have been chemically euthanized, the bodies can't be rendered for meat, even for animal consumption. So you've got 1000 lbs of dead flesh--you can't render it, you can't haul it to the landfill, it's nearly impossible to burn, and it's illegal to bury for the reasons stated above. That's just something to think about.
And, for the record, in the US, horses are killed by a captive bolt gun to the skull, which for all intents and purposes is like being shot in the head. Instantly fatal in 95+% of cases. In Mexico, on the other hand, the spinal cord is severed with a knife, leaving the horse paralyzed and unable to breathe but still able to feel pain as it is hoisted up in the air by chains on its hind legs. Its throat is then cut and it is left to bleed out. And that's where American horses are now going--on a much longer, hotter, cramped ride to a slaughterhouse with no regulations and a disgusting method of killing. Shipment to Mexico for slaughter has increased something like 400 or 600% since US slaughter has been shut down. Good job, animal rights activists!