But you can breed them for slaughter.
No horse may legally be sold for slaughter in the US. If you breed a horse, it clearly is not a BLM horse, besides all BLM horses adopted are geldings and I believe all mares are spayed which would make breeding rather difficult.
There are some horses that are sold in the US and shipped to Mexico for slaughter. These generally go for under .10 a pound. No BLM horse may be transported across the border. At under ten cents a pound you would never cover the adoption fee even if you could somehow send them over the border.