Because it is not just about being fired, it is about concern for Angel and also a ruptured relationship. She has experienced Angelus, but this is Angel going dark and it is shocking and painful as it was never just a job, but rather a mission with a paycheck as well as a friendship.
So why isn’t she pissed off? Probably because she is scared, she doesn’t know where this is heading or what it means, and she is genuinely concerned about the path Angel appears to be heading down.
I agree that it's more about the friendship. Angel and she have been friends for a while, and he has supported her emotionally and helped her out of tough situations. Yet he shut her out when the tables were turned, assuming she wouldn't understand without trying to explain. (Wesley, too!)