Why are environmentalists so concerned about us burning fossil fuels and destroying our natural racehorses?

I can understand them being in an uproar over pollution but natural racehorses are bred and born to do one thing --race. When their careers are over, unless they are adopted and kept as pets on a riding type farm, isn't it the humane thing to do to put them down, especially if they had been injured? They can't be used as plow horses or to pull the Budweiser beer wagon, right? So what's up with that?