Remember Elizabeth Dole's comment about Hillary Clinton when both of their husbands were running for president? "I can't say what she is, but it rhymes with 'witch'."
This question is as old as feminism. Why is it that strong, assertive, and most importantly, successful women are called bitches? Why is it that there is no masculine equivalent to that word? Because a strong, assertive, successful man is just a man!
I think Tina Fey may have been taking part in a time-honored tradition: reclaiming an insulting word. Back in the 1980's, lesbians decided that the word lesbian was not an insult, but a neutral descriptor. And dyke? A Dyke is a strong, assertive, political lesbian. If I take that word, you can't hurt me with it. At least that's the idea.
So, for me, Tina Fey was claiming a word that has been used and used and used against women and saying, yes. Yes, I am a bitch. I am a strong, assertive, political woman.
Oh, yeah, and she was being funny, too.