Because of the way it was explained by her daughter, Palin doesn't have to answer any questions about the engagement. She "didn't know" about it so there is nothing she can offer. All she can so is offer her blessings, which she did.
If you wanted to be really cynical you could suggest that Palin wants the marriage now so the entire sideshow of her single mom daughter is settled with Bristol's marriage is old news by the time she runs.
“Exit question: Some Palin critics are speculating that the Bristol/Levi tabloid wedding announcement was secretly blessed by Sarah.”
What If the whole Levi Bristol breakup was planned by the Palin clan to keep Sarah’s name in the Public eye?
“There’s no such thing as Bad Publicity”
I couldn’t believe it when I read that they were getting back together after some of the most viscious stuff Levi said.