They don't have to. The singular monotone reaction from the top echelon is message enough. I'm no fan of Trump or his policies, but neither am I a fan of an environment where it is assumed we all have the same political views. For a company of over 100k employees, an event like that can't be seen as inclusive.
I probably wouldn't feel welcome at Fox News. Can I force them to become more liberal in the name of "diversity of thought"? Or does that meme only work when it's trying to promote people on the right?
Google can do whatever they want as long as they are honest about it. If they want to call themselves “The progressive search engine” then at least they are being upfront about it.