Simply put, humans are more important than horses. This is widely accepted throughout the world.
One could argue that the horse benefits as well. Should the horse whose immunity has been built up through invenomation be grazing and get tagged by a cobra or a krait (or the other two species of the area that are being used) he would be more likely to survive the bite. For curiosity's sake, I wonder how many horses are killed by snakebites. You never hear those numbers, just human deaths. It makes me wonder.
Another could say poor snakes. Dug out of their holes, put in bags and pots and milked for their venom. But they aren't as cute and furry as horses and are the direct reason the horses are being used in the first place.
We do it to our horses with different substances every time we vaccinate our horses. define:Vaccinations - Google Search