I have to err on the side of caution and say no, just as an employer can't fire someone for their religious beliefs (which are arguably just as inflaming as the viewpoints people take issue with in this thread).

No right is unlimited, even if it's in the constitution.

For example, libel, slander, and harassment may all take the form of "speech" but are unquestionably firing offenses. Those are also criminal offenses because the balance of harms swings away from the person who's free speech rights might be violated, toward the person who's own rights are being violated due to the exercise of free speech itself.

As that old adage goes: My right to swing my fist ends where your nose begins.

Unlike in many countries, in US in most states employment is "at-will" this sounds nice and friendly but it means officially they can let the employee go for any reason at any time unless that reason is one of discrimination due to a limited set of categories. Of course people are discriminated all the time based on religion, race, sexual preferences, age, gender etc, it is just that is not the official reason. It can always be molded into some "performance review", "downsizing", "business goals" etc. kind of thing. Proving otherwise requires an expensive lawsuit looking for patterns over a longer period of time and candidates or requires someone to blow the whistle and say "oh when the CEO say 'culture fit' we really mean they are of a wrong race or gender, everyone knows that around here..."

You can't be let go for being a whistleblower or someone who wants to improve hiring practices. The author has grounds for a lawsuit and was likely fired illegally.

But they can always find another reason to let him go. "Restructuring" or some similar thing would work. They just fired him, I presume the lawyers looked at it and probably didn't see much of a risk.

Doesn't pass the sniff test. His memo was open, notorious, and even received a response from senior management.

More likely than the lawyers saying the firing was legal, is that Google decided the cost of an illegal firing was less than the cost of keeping him onboard.