The regional and topical listings have different criteria. Just because a site is listable topically doesn't mean it's not listable regionally. Conversely, just because a site ISN'T listable topically ... still doesn't mean it's not listable regionally.
Yes, "suggest a site to the one best category" is what the policy says. BUT the topical/regional dichotomy mans that suggesting a listable site to both kinds of category actually helps the editors without opening the door to shotgun spam. So it's tolerated -- accepted -- actually appreciated, if the site really is applicable in both places.