Then why the fires of Phlegetos in her eyes?
If Usizil were a Something of Viserys (Cleric or so) then I could see an indirect connection, like Viserys' power being filtered through the Erinyes to produce such a result, but that is not the case.
Maybe she is something like a concept-Warlock? Warlock of Lawful-Evil, as filtered through Usizil's knowledge of it, which is still that of Hell of course, after millenia down there. We know that at least one Erinyes has become a Paladin of Tyranny, of Lawful Evil as a principle force, so maybe this is a similar connection? She is still a creature with the Lawful and Evil subtypes, a Devil in all but homeplane, so that counts maybe.