General Discussion

Unionized workers are not allowed to take over the boss's office. Employees can discuss unfair labor practices with each other while they are on break. Employees cannot dictate who their employer does business with. Engaging in these activities allowed an employer to terminate an employee.

From the NLRB

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.

Strikes for a lawful object.Employees who strike for a lawful object fall into two classes “unfair labor practice strikers” and “economic strikers.” Both classes continue as employees, but unfair labor practice strikers have greater rights of reinstatement to their jobs.

Unfair labor practice strikers defined.Employees who strike to protest an unfair labor practice committed by their employer are called unfair labor practice strikers. Such strikers can be neither discharged nor permanently replaced. When the strike ends, unfair labor practice strikers, absent serious misconduct on their part, are entitled to have their jobs back even if employees hired to do their work have to be discharged.

Furthermore, Section 8(b)(4) of the Act prohibits strikes for certain objects even though the objects are not necessarily unlawful if achieved by other means. An example of this would be a strike to compel Employer A to cease doing business with Employer B. It is not unlawful for Employer A voluntarily to stop doing business with Employer B, nor is it unlawful for a union merely to request that it do so. It is, however, unlawful for the union to strike with an object of forcing the employer to do so.

Examples of serious misconduct that could cause the employees involved to lose their right to reinstatement are:

Strikers physically blocking persons from entering or leaving a struck plant.
Strikers threatening violence against nonstriking employees.
Strikers attacking management representatives.
https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/employees/your-rights-during-union-organizing
https://www.nlrb.gov/strikes

The AP has a more detailed article:
I believe Google had the right to terminate them.
I don't believe the bit about "non-participating bystanders". If you are at work, how can you be a bystander?

Google fires more workers who protested its deal with Israel

Google said it fired the additional workers after its investigation gathered details from coworkers who were “physically disrupted” and it identified employees who used masks and didn’t carry their staff badges to hide their identities. It didn’t specify how many were fired.

The company disputed the group’s claims, saying that it carefully confirmed that “every single one of those whose employment was terminated was personally and definitively involved in disruptive activity inside our buildings.”
https://apnews.com/article/google-israel-protest-workers-gaza-palestinians-96d2871f1340cb84c953118b7ef88b3f