Agreed, in general. As someone who has worked retail, I'd just ask them to leave once and then call the police. There's no need to mediate this situation.
Yeah, if one were really going to support militant action against the meat industry, it should be way higher up the chain where the decision makers and infrastructure are. There are a lot of bigger consequences to doing that, which is why timid people usually just harass consumers.
Also, I'm not a vegan. I don't advocate for harassing people that don't share my ideology, either. I'm just opposed to vigilante "justice."