Its a game. The same can be said for murdering & viciously killing other players in this game. Yeah it seems all f*cked up in the end but if it actually isn't being commited in reality then i don't see the problem and this coming from an animal lover. And before society become civilized, animals and humans were fighting til the death and was done as a means to survival of the fittest.
They had a hard enough time just getting animals in the game. Dogfights would be a lot harder, not to mention probably more screwed up than the torture scene as it's voluntary and for enjoyment.
Personally id love too see a dog rip another dogs throat out. Rockstar has always done controversial thing's why not dog fights.
But think of it this way:
Killing people in GTA isn't a big deal. Why? It was made for killing people, and you can tell by the undetailed deaths.
Now, with the addition of dogfighting, how detailed would it be? Would it be gruesome, or would it look like two wolves from Skyrim fighting each other (really weak animations, it's kinda funny to be honest)?
Now, organized dogfighting, like if they made it a thing like arm wrestling or darts - I'm not for it, and chances are that crazy animal rights group would have it shut down (or at least try). If it's something that happens to go down in the streets, then whatever that's fine by me.
Gotta keep it simplish, you know? Even in GTA there's such a thing as too violent.