The moderator did not call Romney a liar. Romney stated something about Obama not calling the attacks in Bengazi terrorism, Obama said it wasn't true, and referred to 'the transcripts', Romney then repeated his claim about not calling it terrorism. It was only then that the moderator said 'he did'. It is not unlikely that by then her team had already pulled the said.
Other than you I think it is very good that politicians get confronted when they lie immediately after they have told one. If you let them get away with lies and wait untill the talking heads and fact checkers do their work the lie already has stuck in far too many people's mind as a possible truth.
If you google the moderator's name you will find that ahead of the debate BOTH campaigns were concerned about the moderator not sticking exactly to their set of negotiated rules. I think that's a good thing; politicians should be affraid of the press, not be able to treat them as their lapdogs.
http://www.youtube.com/watch?v=SK-NzSNYaJ0