Sorry, for Robb Stark and/or Stannis (since it's not for Renly, and Balon just hijacks the chaos)
Well, for Stannis, he certainly has just motives for rebelling. Cersei murdered his brother and is attempting to pass off an illegitimate heir. By the laws of Westeros, he’s the next heir, so he’s not wrong to fight for his kingdom. Not only that, but many of the things he’s proposing are both fairly radical and objectively better for Westeros. Ridding the royal bureaucracy of bad actors like Varys, Cersei, and Littlefinger are good for the country.
As for Robb, well, look at what he’s dealing with. He’s fairly convinced that the Lannisters have murdered Eddard falsely, and this is the third Stark in two generations to be murdered by the Iron Throne. Unlike Stannis, Robb has no reason to believe that Joffrey isn’t the legitimate seated king as eldest male heir of Robert’s body, so he’s not installing Stannis. Rather instead, he’s fighting for independence against the Iron Throne which has murdered his vassals and unleashed a campaign of terror against the Riverlands, a direct casus belli that Robb acts immediately on.
Thanks for the question, Anon.
SomethingLikeALawyer, Hand of the King