It's a little bit of a silly season story, but of course Romney wants Republican voters to think he agrees with the treason charge against the president. Romney needs lots and lots of this kind of thing.
He's shown an aptitude for saying and doing anything to win, including lying with astonishing frequency and running the most cynical campaign ever. So it's no shocker that he let that woman's laughable accusation slide. In fact, Romney has lied about the president on a daily basis -- so the accusation of treason is minor in comparison.
And so this won't be the last time this happens.