I think it's a necessary evil.
People in Australia are trying to ban the brumby cull in QLD. Brumby cull to kill thousands in Carnarvon National Park | Courier Mail
Sure they could probably think of a better way rather than having them all running froma helicopter, but it's still something that needs to be done.
It's the same crap we get from people overseas when we have to cull the kangaroo population. Or when we had to cull a heap of camels in the desert because they were over populating.
It's just not possible for every horse/other animal to be left alive. That's why there's natural selection. It's not nice, but it's better than domestic horses starving in a crappy paddock with no care, just like the cull is better than the brumby's exhausting the food source and starving in the bush.