She could have declared the subject off Bristol off limits at the beginning of the interview.
I'd say that even if Palin offered that information without being queried about it makes sense - first of all, it's probably true.
Secondly, disclosing that kind of information isn't any more damaging than anything else she's been through. Third, being forthright and up front about personal issues of that sort tends to diffuse and otherwise take the spark out of personal attacks. I'd be the first to say, however, that the moonbats will apparently stop at nothing until they think they have something slanderous and damaging to say abou either Palin or her family.
Fourth, Palin admitting that she wasn't aware of her daughter's sexual activity makes her seem real - a regular mom with real teenager issues at home.
That kind of thing tends to endear Palin to regular people, who are those who aren't threatened by her.
Moonbats ain't "regular" people. They're clogged-up little shits of people with terminal hemmorhoids.