A right to do something does not extend to taking over someone else's means to do it. You are free to talk to someone in person about your wages. You are not free to use my equipment to do it. Or to bill me for your time spent discussing it. By your logic, a union organizer should be entitled to photocopy union organizing fliers on a company copy machine.
But you are not.
Notice that I would say that printing union fliers on company printers is fine to not allow, we can have different areas where we draw lines. I think sending an email to employees that a discussion is happening about union is fine. You seem to be on the camp of since the line can be drawn anywhere it should be drawn on the extreme that benefits the employer.
As if me asking Jenna over slack what her bonus was is incredibly taxing on the company for some reason. The only reason that you don't want this to happen over email or company slack/messaging is to prevent this communications to happen at all.
I'm stating facts within the law. You are stating wishes, made unbelievably clear by your saying that I'm trying to get you to side against your data and interests.
Laws don't operate by saying that "unions are beneficial to society and to me" and so therefore it must mean that you're allowed to use other people's equipment to do what you want. Once you bring in that argument, it's clear you've lost.
I guess it's true: "don't argue facts against a man whose livelihood depends on ignoring them."