Zuko didn't accomplish anything, that's the point. Remeber way back when..."And your brother Zuko is a failure, I have a task for you." She still has the duty of bringing back Zuko and Iroh, she let Zuko eat out of her hand, and now he trusts her (or does he, oh well, its beside the point). She wants to make him feel like she's the good guy, so she told him he could go home, when in all reality she's just going to hand him over like she was supposed too, excellently played on her part.