I'm a bit torn on this, perhaps leaning a bit on the side of the firing being at least morally legitimate.
0) Her behavior was way out of line. No question about this.
1) There's no company out there that should be expected to look the other way when your employees are giving them a black eye on official channels. (And we can quibble about this, but when you're running officially blessed Q&A sessions and state your employer's name up front, it's "official enough" for most purposes)
2) Doing nothing or "handling it internally" would not have resolved the black eye. If anything, radio silence would have made ArenaNet look like they tacitly support rudely shouting down perfectly reasonable questions.
3) Making a public statement about how they don't support the words said (usual corpspeak PR nonsense that everyone can smell a mile away) would be recieved as cynical and noncommittal, possibly even making the brand perception issue worse.
4) Putting your employer into a lose-lose situation by your actions is generally a firing offense.
F2P games live and die on engagement in a way that paid titles do not, which magnifies the issue somewhat. Come to think of it, to memory, most firings that made the games media circuit that I can think of were F2P companies. Riot did it to someone making distasteful comments about a particularly infamous player.