Mayer was probably too new and too scared to deal with this, especially when she had to think about her new baby.
My point is it has less to do with "Yahoo being scared about being destroyed by the US government" and more with Mayer's own personal fear of the US government, and easily caving to such threats, whether it was because of her baby, because of weak character, or because she was inexperienced as CEO didn't know how to handle this.
EDIT: Also Yahoo's response to the story is such bullshit:
> “Yahoo is a law abiding company, and complies with the laws of the United States.”
What's the law you're abiding with that says you need to implement such a backdoor? Name it. Even the FBI named the law or laws it thought would help it force Apple to put a backdoor in its phones.
So if Yahoo can't even name the law, then that law or the gag order saying they can't name it are unconstitutional. Period. It also proves once again how easily Yahoo caved.