At will employment doesn't matter here. Even with at-will employment it is illegal to fire employees for some reasons. For example, it's generally against the law to fire employees for discussing their work conditions.

I think the employees will have a difficult case here, but it is conceivable that some of them might. For example I could see the NLRB ruling in favor of an employee who was fired after for expressing concern about a project with Israel, but I think the employees who staged a sit-in probably don't have a case.

Well you can't fire people for political reasons. It's a protected category. So the crux of it will likely depend on if there's similar protests from opposing viewpoints that were not dealt with similarly. Eg. If there were any counter protestors at the same event that weren't also fired that's a reasonably clear cut case of firing a group for political views.