I've been paying some attention to this topic ever since it came to my attention about 15 years ago. As with everything else, there's at least two sides to the situation.
Emotion- I see emotion on both sides of the issue. Anti-slaughter folks see "all the pretty horses" as was mentioned earlier. Pro-slaughter folks... well, in my part of the country, a good many farmers see horses as being just like cows (when considering the horse's anatomy/physiology/psychology, they're really not so much alike), yet so many people across the country see it as an individual property right issue. Myself, being a horse owner, I manage my horse as I'm taking care of a good friend... my horse's health and safety are my responsibility. I thought everybody knew a cowboy's best friend is his horse.
Price of horses- At the horse auction I used to go to, starve-outs were being offered for sale for the same money as sound horses. Crazies were drugged and sold for the same money as good solid dependable horses who'd make their owner proud. $300? I hardly ever saw a horse sold nearly that cheap.
Mustangs- I've heard from more anti-mustang people... either they're wearing out the land or they're crazy. I know they're not crazy... no crazier than a domestic horse, and many times less so depending on how they're treated by humans. Any horse can be made dangerous by mutual distrust. (By now, ya'll have realized what my online handle is.) As for ecology, it's more that they're competing for forage the ranchers "must have", and it's not that the ranchers don't need it, but rather that I've heard of ranchers trying to pack too many cows on the same range too.
The French- Who cares? Really. If they want horse meat, let 'em eat their own horses, but they can leave me alone. It ain't my responsibility to sell them horse meat or any other kind of meat.