Because part of the contract you mention involves adhering to employment law, which generally says that companies cannot fire people for union activity.
Edit: Why are people downvoting me for asking for a source?
> 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/rights-we-protect/whats-law/employees/i...
I imagine that Googlers are generally permitted to socialise during work time, so they also get to unionise during work time. I even imagine they sometimes use work-provided services to organise non-work-related activity and this is permitted by the company.
That said, I do wonder if the fired workers could argue that they were merely taking an unscheduled break (permitted under Google's flextime policy) when demonstrating.
Nobody was fired, supposedly, in relation to that demonstration.
According to Google, but the workers are claiming otherwise.
> but it’s quite possible that they had that day off.
Indeed. In fact, with Google's flextime policy, can't they simply choose to have that day off?
Intermittent strikes, for example, are not.