Each year about 50,000 horses

are stunned by a four-inch retractable bolt shot into the brain, then hung up by a hind leg to have their throats slit so they can be bled out. The method, required by the federal Humane Methods of Slaughter Act, is the same used for beef cattle....Rescuers will buy sick, lame, malnourished or blind horses at auction to either rehabilitate them or have a veterinarian euthanize them, with the carcass then hauled away to a rendering plant for disposal. Euthanasia and rendering can cost an owner a few hundred dollars, which explains why some refer to slaughter as "the poor man's euthanasia."

After all,

the horse was going to be put down anyway...and the method of slaughter has been federally sanctioned as humane...why shouldn't the meat go to feed people rather than to "a big hole in the ground?"

But

Texas prosecutors are threatening criminal charges against the last two equine slaughterhouses in the United States under an obscure 1949 state agricultural statute unearthed by animal welfare activists. Meanwhile, a bill being drafted in the U.S. Senate would make slaughtering a horse for human consumption a federal offense, even if the meat is for export only.

Although the laws would not prevent slaughterhouses

from continuing to slaughter as many horses as they wanted, in the same manner, for premium pet food or other byproducts.

So it's basically all about not permitting dead horses to be eaten, even though they're going to die anyway. Meanwhile, other meat can be eaten by humans.

update
whoops, this was from Tamara Jones.