It would be a little weird if legal travel was more of a hindrance to powers than illegal travel, but it is a thing that IMO really needs some fanwank, because, as you say, it's not like we hear this about the 'bad' Dixingians that come to the surface.

(Maybe it's a comment on how bureaucracy and the proper forms make everything harder, including use of your power. *g*)

On the other hand, maybe the 'bad' Dixingians are also constrained, or they wouldn't have a super long lifespan on the surface because they burn through their energy too fast... On the other hand, there's butler Wu, who's been around for a long time... IDK, IDK!