First, I think this is a pretty just sentence. Five years is plenty for killing a few horses. I don't know if he will try the same thing on humans (I doubt it) but you can't punish someone for something they might do in the future.
Second, there would be NO black market for horse meat if you could buy it in a grocery store. There is a huge ethnic population in Florida and most of them are from places that have no tabu about eating horses. It would be an economic benefit for everyone involved if horses could be slaughtered and sold for human consumption.
Finally, I have to say what a bunch of idiots these guys are. If they had gone to a sale barn and bought a horse for $50 and taken it out to the everglades and slaughtered it nobody would have given a hoot. Instead the ignorant fools had to destroy someones property.