There's several arguments Dionysus wants to make, with regards to Ariadne's point about his uncle. Yes, Pluto doesn't like allowing mortals to come back, but he let Dionysus bring Will back. ...Granted, he did technically trade Xanthias for the opportunity. Pluto didn't know Xanthias was planning on staying, after all. He could also argue that the nature of this place is so utterly removed from anything his uncle has power over, or else Grantaire wouldn't be here right now. But he doesn't argue, he just wraps his arms a little tighter around her.