This is an old labor issue, but it used to revolve around handing out union flyers at work and posting on company physical bulletin boards.[1] That's generally allowed by law for labor activity, even if the employer doesn't like it. How this translates to online activity isn't entirely clear yet. But courts will make that analogy.

An employer who tried to pull this in 1970 would have had a picket like outside the door the next day. Union truckers wouldn't deliver to their loading dock.

[1] https://www.jacksonlewis.com/resources-publication/employers...