Why did Baelfire have to go back when the curse was destroyed?

We don't actually know that Neal or August ended up back in the Enchanted Forest, although Regina didn't say anything to indicate that anyone but Emma would stay behind. (If they did go back it will be interesting to see if they reverted to the age they were when the curse hit; unlike the people in Storybrooke, they've been aging naturally the whole time.) We'll obviously find out when the show goes back on-air.

Assuming that they did, the explanation that is given in the mid-season finale centers around the fact that Emma is special not because she came to Storybrooke through a portal, but because she's "immune" to the curse effects. Rumple engineered the curse such that Emma was a built-in safeguard to eventually break the curse. Baelfire, on the other hand, is the entire reason the curse exists in the first place. Rumple put the whole thing together specifically to get himself to Earth to get his son back. And Pinnochio was sent to Earth because of the curse, so he avoided being trapped in Storybrooke, but otherwise has no special immunity to its effects if he were caught in the spell.

Presumably, the spell Regina uses to stop Pan's curse is closely related to the curse itself, so it will likely have many of the same "features" of the original. So, by the vague and convoluted laws of Enchanted Forest magic, it does make sense that rolling back all of the effects of the curse might also affect Balefire. Indeed, it should also effect Emma, since she was born in the Enchanted Forest. But her special relationship to the curse-spell gives her unique protection from any of its effects.

So I know this is a bit late to reply since OUAT is on season 7 now I ended up here wanting to read fanfics and this is way too interesting not to say anything....now while I believe Emma may have some immunity to the curse...I believe the reason why Bae/Neal was affected, is because he was in storybrooke while the reset happened. While Regina did say the reset would be as if the curse never existed, the reset only affects those within the reach of the town. We see that Henry's camera was still in New York. Henry was worried that he would be alone when the reset happened as it would take back everyone who was born in Storybrooke. Emma escaped because she drove across the town line, creating a barrier between the purple cloud which held the curse and her. The only person that wouldn't have been affected was Henry. If Emma was in the town when it happened, she would have been taken back too. Similarly, if Neal left with Emma and Henry he would have been safe. Regina could have given them fake memories of she and Neal raising Henry(in Boston) and that camera would still be in NY. There was no real reason other than that's the plot they were going for why Neal had to stay behind. Gold was already dead, so it's not like he would never have let Neal go. He should have gone with them...only Regina needed to let go of Henry. Probably they already knew they were going to kill him off. Henry didn't even get to see his father after they came back from the year away. The writers just knew Emma and Bae wouldn't be endgame and the fake memories plus the memories the three would make together in that one year after would only develop Neal and Emma's relationship. And this in turn would make it harder to accept her with Hook. So that could be why. Neal staying back is in no way connected to the terms of the curse...just plot reasons you know.

I very much agree with user98705. Rumple created the curse so that he could come to our world and find Baelfire/Neal, but the actual curse itself had nothing to do with Bae. It was merely a tool for Rumple to find his son. So it doesn't make any sense that Neil wouldn't have been sent across the town line with Emma and Henry with his own fake memories of them raising their son together.

Other than the fact that, that's not where the story was headed. I believe that the writers already knew that Neal was not going to survive season 3, and that Emma and Hook would be starting their love story. For that matter, Hook also shouldn't have been affected had he also crossed the town line before the curse hit, because he was protected from the original curse and only came to Storybrooke many years later with Cora. But, again, the story needed Hook to be sent back with the others so he could go and get Emma's memories back to her so she could come back to Storybrooke.

Neal/Baelfire should have gone with them. If the reset only affected those within the town then theoretically he should have been able to go with them. They could have had a new set of memories & been a family from beginning. I really do not see why they weren't given that chance. It literally made no sense why the writers didn't give them that chance. Even Emma suggested it and Neal/Baelfire said no that he had to go back to try to bring back his father. That I could have understood. It made sense why Snow & Charming couldn't even go with them even if they left at the same time so they weren't in the town when the reset happened. As they were brought over with the original curse. But Neal/Baelfire didn't have to go back. I really don't think he was brought over from neverland with the original curse. He must have escaped from neverland another way. But then again it is also never explained how he came back to our world from neverland. Same as it's never explained how Dr Frankinstien ended up in Storybrooke as well. He was in his world when the curse struck surely as he went back to use the enchanted heart he got from Regina to resurrect his brother which he did and failed. But as far as we know he never went back to the enchanted forest. As far as we know he was the only one brought over from his world. So that stands to reason if the curse was able to pull just one person from another realm then I guess it could have done that with Neal/Baelfire in neverland. Then again though if that was the case why was Neal/Baelfire never brought to Storybrooke like Dr Frankinstien was.