IANAL, but as far as I know, the line is "intent", and courts of law are empowered to infer intent from actions. > things that could get someone fired in compl

IANAL, but as far as I know, the line is "intent", and courts of law are empowered to infer intent from actions. > things that could get someone fired in completely union-unrelated situations, are actually protected if done in service of unionization? If you preferentially fire union organizers and preferentially ignore the infractions of non-union-members, then yes.