It was a silly joke they keep laughing about over dinner.
According to OSP, Hades got permission from Zeus to kidnap the girl, which more or less how things were done still. (Yes, we had ritual kidnappings.) It was Demeter and her codependency on her daughter that caused problems. So Persephone is essentially doing caregiving service for mom every othern six-month interval.
Old-timey pre-Hercules Disney paints Hades as kinda Satany which informs 20th century interpretations. Curiously in the Percy novels, Hades is more true to Hellenism, but gets a touch of the diabolic for the Lightning Thief movie.