The NLRB rules actually say you can’t be punished for talking during work time about union activity, either. If you’re allowed to talk about other non-work subjects, you can talk about organizing. They can pass non-discriminatory rules that say “no personal conversations on the clock,” but if you can talk casually on the clock, you can talk about unions.

It’s different for emails though since that could be construed as doing non-work activity on the clock, but if you’re on an assembly line and you’re just talking about unions while still working, it’s illegal for them to punish you.