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.

1: https://fas.org/sgp/crs/misc/95-815.pdf

2: https://www.nlrb.gov/rights-we-protect/whats-law/employees/i...