CONTEXT, shall we?
Luna was incredibly scared confused, and vulnerable at that point. She was probably anticipating her sister striking her or binding her up again after everything she did, and was genuinely surprised that she was being so forgiving to her.
Remember, Luna only turned after she decided to rule over her own emotions, the same emotions which contributed to her own downfall.
Sure, it may seem like Luna did not have much of a choice due to the apparent imbalance of power, but that doesn't really matter, because as far as Luna was concerned, it looked an awful lot like she did have a choice. Celestia wasn't holding her magical or political power over her, she was holding her love over her. And since Luna wasn't magically bound up, she probably could have chosen to flee at that point and hide, wallowing in her own patheticness.
In the end, Luna's reformation was emotionally poignant, because she wasn't regretful that she lost, she was regretful that she hurt the sister she loved, who also loved her just as much.

After Sunset Shimmer gets blasted into the crater, Twilight walks up to the edge of the crater and stands over her. She says "Tonight, you've shown everyone who you really are. You've shown them what is in your heart." in a reprimanding tone, which then prompts her to break down into tears on the spot. Twilight does not act compassionately toward Sunset, she acts scornfully holier-than-thou. As such, Sunset has no illusion of having a choice; you could say that Celestia made an illusion for Luna, but you can't say that Twilight was doing the same for Sunset.
Of course, one might argue that harshness is needed for a vindictive asshole like Sunset Shimmer, but I'd argue that guilt tripping would have probably worked better in this situation. "What were you thinking Sunset? You hurt all these people, and now you are sitting in a crater with nothing left. Was it worth it?". Then, send her back into Equestria for therapy sessions with the real Celestia, the pony with whom she has the issues in the first place. Who knows? It might've just worked. Anyway…