So, causing undesirable mental states in other people shouldn't be allowed? That's a very slippery slope. These students aren't literally going and assaulting immigrants on campus. They aren't making citizen's arrests or throwing paint on people to 'mark' them for a hunt. What they're doing is theater. They're creating a simulation of something they would like to happen because they feel strongly that illegal immigrants should not be allowed to stay in the country. That's a form of political protest and we are obligated to protect it even if it makes people uncomfortable. Should abortion protesters be banned from showing images of fetuses because it might make mothers feel sad?
Should anti-hunger organizations be banned from showing starving children because it makes obese viewers feel guilty?
Or, something more akin to this situation - should we avoid reminding drug users that they could get arrested because that paranoia might upset them?