Random speech doesn't become any less random just because it is federally protected. On the contrary, it actually becomes less federally protected if it is random, because federal protection of speech requires specific time, place, and manner.
At a broader level, federally protected speech in the US isn't absolute; the Supreme Court has ruled a number of times that restrictions can be placed on organizing protests, such as you can't protest in the middle of a busy freeway at rush hour blocking traffic and use "my right to protest is federally protected" as an excuse. There's more info here [1] (PDF warning; go to page 9).
More specifically to unionizing, employees are allowed to discuss unions but employers are allowed to place some restrictions on when/where such discussions can take place. From the NLRB [2]:
> Working time is for work, so your employer may maintain and enforce non-discriminatory rules limiting solicitation and distribution, except that your employer cannot prohibit you from talking about or soliciting for a union during non-work time, such as before or after work or during break times; or from distributing union literature during non-work time, in non-work areas, such as parking lots or break rooms.
Federally-protected content isn’t random speech.
There are reasonable arguments on both sides. Looking forward to seeing them play out at the NLRB and in the courts.
The only solid takeaway at this time is, if you think this is black-and-white, you’re missing salient detail.