Never worked for one of the big tech firms, but I have been in the working world for ~16 years and one of the few things anyone that has been around for awhile can and will agree on is you don't talk about salary within earshot of the boss, you don't badmouth company decisions within earshot of the boss, you don't talk about politically charged topics, and you certainly don't combine 2 of those 3 and protest company decisions on politically charged issues literally in the office.
You also don't do those things on company provided equipment, software, or services. If you want to bitch about something the company is doing, you go out to lunch or do it after hours, preferably without written or video evidence.
While I think it is gross that Google fired them for this, given the history of the company almost encouraging such things, I can say these people just got a hard lesson that most of us learn about the corporate world long before we make it to working for the likes of Google.
Rightly or wrongly freedom of speech, assembly, etc protects you from the Government, not your boss. And your boss is a petty little ego maniac that controls your livelihood, so best to stay out of his gaze on matters you know he/she would view negatively where at all possible.