[bpodgursky]

It's generally legal to fire employees for "free speech" activities done outside the workplace.

More morally hazardous of course, but I don't think anyone would really even challenge the cause here (it's trivially easy to argue that this behavior shows they would endanger workers within the workplace as well).

[sudosysgen]

Except if that free speech is an attempt to organize.

[ForHackernews]

Even in the United States, it's typically illegal to retaliate against employees for organizing: https://www.workplacefairness.org/unions-retaliation