I think that's just the necromancy thing.  Enthrallment doesn't specify a mortal or a human, and Harry explicitly accuses Victor of breaking it in the 1st book for having summoned and bound the trenchcoatted toad demon. (Morgan also accuses Harry of the same, claiming that it is against the 4th law)
I think he also reiterates the point in book 1, book 4, and book 10, saying how careful he is to imbue the words he uses to call Toot with as little will as possible to avoid breaking the law.

Calling something (via summoning, etc) is fine, as the creature is not forced to do anything.  The summoning typically will peek the curiosity of the being you call.  As long as you bargain with it, instead of force it to act, there is no violation.  As for Harry binding Kravos' ghost he isn't enthralling the being, and he isn't dominating it.  He is altering the metaphysical reasoning behind its existence.  The Nightmare, like all ghosts, has a Purpose for existing, as Harry explains later in the book.  What Harry does to Kravos is redirect that purpose from EVERYONE that took him down to ONLY HARRY.  Kravos is still free to do as he wants (as far as ghosts are able) and would be perfectly fine committing violence against anything in his way, but will fixate on Harry, at least until Harry is dead.  It's the spiritual equivalent of Harry running ahead of his gang with a target on his back.