I think you’re wrong... It’s only legal if they prohibit all non-work discussions. So if they also block discussions about politics, sports, music, etc they can block talk about unions.

But they can’t target union talk specifically:

Companies may prohibit workers from talking about nonwork issues in work areas during work hours. ... In other words, a company can prohibit extraneous conversation, but may not prohibit only union discussions while allowing employees to talk about anything else they wish.

Source: https://www.nolo.com/legal-encyclopedia/shop-talk-rules-unio...