To let nature cull means to let them starve to death. I don't understand
why people get so upset over a human killing a horse (as opposed to a cow, deer,
moose). Go watch a "wild" horse live it's whole life in a crowded
pen for a few days and maybe your opinion will change. Many countries eat horse
meat, just because americans don't doesn't make it wrong.
We round them up because it improves grazing for ranchers in the same areas.