Your understanding of the law is incorrect. If that happened to you or someone you know, you should contact the NLRB.

>You have the right to form, join or assist a union.

>You have the right to organize a union to negotiate with your employer over your terms and conditions of employment. This includes your right to distribute union literature, wear union buttons t-shirts, or other insignia (except in unusual "special circumstances"), solicit coworkers to sign union authorization cards, and discuss the union with coworkers. Supervisors and managers cannot spy on you (or make it appear that they are doing so), coercively question you, threaten you or bribe you regarding your union activity or the union activities of your co-workers. You can't be fired, disciplined, demoted, or penalized in any way for engaging in these activities.

>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. Also, restrictions on your efforts to communicate with co-workers cannot be discriminatory. For example, your employer cannot prohibit you from talking about the union during working time if it permits you to talk about other non-work-related matters during working time.

https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/employees/your-rights-during-union-organizing

edit: add emphasis