If you think that's okay because of the views this engineer holds, you're a terrible human being.
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.
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.
I guess you don't work in the United States? Because it's certainly fine for him to be fired for this.