If the precedent is "a talking to" with future offenses resulting in a firing, then smart Google employees will realize that they can create an internal activist email list, get new hires to join it, and then when an opportunity for activism presents itself get an employee with a clean record to perform the activism, rinse repeat.
Of course the message in this case wasn't terribly harmful, so I'm sympathetic to the argument of "it's not such a big deal". But the problem is, Google doesn't want to wait for the big deal (for example a chrome extension change to search exec's TextAreas for keywords a "whistleblower" might want to report on) to actually happen and then fire them. They want to prevent employees from thinking they should ever try doing that in the first place which means putting the foot down before the inevitable happens.
Speaking of provocation: I suspect she might have done this to intentionally get fired. It wouldn't be the dumbest play, she's super young (21), if she doesn't want to code anymore she could use the exposure to parlay this into a new career, and if she does want to still code, she probably made some new supporters and her detractors will factor in youthful indiscretion. Wild speculation sure, but, you just read a big press article about it... Where do you think the tip came from? This isn't the kind of move you make if you intend to stick around at a place.