Was this debate ever exclusive to the United States? I'm sure you've noticed that in Canada we have a pretty active slaughter industry, complete with our own anti-slaughter movement. I'm pretty sure I have every right to express my opinion about something that happens on an international level.
As to having an opinion on specifically U.S. Slaughter practices, as long as unwanted American horses are being shipped up here for slaughter because there's no outlet for them in the states, I'm happily going to voice my opinion on it