I couldn't send my old girl even if that was the case. Honestly I think it would be too traumatic for her to be taken off property and brought to a slaughterhouse. But this is a 30 yr old horse that hasn't been off property in like 10 years or so (beyond the occiasional trailride from the house). I'd be worried about any trailer ride for her at this point due to her balance and arthritis issues. But again that's my specific case/horse and doesn't have anything to do with your decision.
When we put my gelding down, my dad shot him and we had the renderer come and pick him up. As far as I know his body was used. But really even if we had buried him, his body would've (eventually) been used by the plants/bugs/etc in the soil. But I don't think that's really the point or at least it isn't for me.
For me it's the whole "personal freedom/choice" aspect. I don't like a bunch of strangers telling me what I can or cannot do with my animals. As long as I'm not abusing them (starving or physical) it's my choice. Personally I'm a big baby and I treat my animals like a part of my family w/ concessions to the needs of their particular species. But others are not like that and that's just fine.