True, but Tyrion is still Joffrey's uncle, and at least book!Joffrey is only 13 or so. And even though Tyrion is physically weaker than Joffrey, Joffrey can't really fight back because of the social situation (I mean, come on, what nephew gets away with beating up his dwarf uncle?). I mean, I can see that there's a slight difference, but that difference isn't big enough to call one situation horrible, unacceptable domestic abuse and the other one cool and badass and perfectly fine.