Oda always does everything with a purpose. If we look back at one of the first feats we learned of Blackbeard, is that he rampaged through Drum Kingdom, and turned it into a nation without a King. This is most likely symbolic of the fact that Blackbeard himself is Royalty, without a Nation, which would mean that his father, was once a king.
It makes sense considering Harold had to sneak into the Reverie, while it seems that Xebec was there as a guest, as it wasn't mentioned that he launched any kind of raid or assault on Mariejois, only that he was there, and that one incident is what earned him a bounty and the governments ire. That means he wasn't wanted prior to the incident where he killed an Admiral, and only Kings and Queens are invited to the Reverie.