How naive you are to think that they could just walk away. Garrosh has a fucking army right there with him. You think he's going to just let Baine take his Tauren and leave? Every time Baine voiced his objections, Garrosh threatened to kill Baine: “If you do not wish to share your father’s fate, I would advise you to watch what you say!” It's also why Vol'jin kept mostly silent: “Dat why I stay quiet. Gotta tink about my people.”
Even Jaina understood that Baine couldn't just up and leave. Baine can't even fully join the Rebellion in 5.3 because he has to get his people out of Orgrimmar first.
“He despises these actions, but to protect his people, Baine has agreed that the tauren will continue to serve the Horde as they are needed. He wishes me to emphasize that at times, this obligation brings him little joy.”
“My high chieftain remembers the aid you gave him and asked me to warn you. He has no wish to see you caught unawares.”
Jaina was overwhelmed at the gesture. “Your high chieftain,” she said, her heart full, “is a truly honorable tauren. I am proud to be so highly regarded by him. I thank him for this timely warning. Please tell him it will help save innocent lives.”
“He regrets that a warning is all he can give you, my lady. And… he asks you to please take Fearbreaker, and return it to the one who so
kindly gifted it to him. Baine feels that it is no longer his to keep.”
Jaina nodded, though quick tears stung her eyes. She had hoped that that night would be the beginning of healing, of understanding, but it was not to be. Baine was telling her, in his typically gentle but firm fashion, that their friendship only went so far—he was not, and would never become, a member of the Alliance. He would stand and fight with the Horde. She understood. She was fully aware of how vulnerable the tauren people would be if they stood against Garrosh now, and she had no wish to see them come to harm.
It's the same shit with the Forsaken and Gilneas. They didn't want to attack, but Garrosh threatened to burn UC to the ground if they refused. He had a full army right there with him to do it too.
Look what happened to the Trolls. They walked out of Org and Garrosh sent Kor'kron to subjugate them on their islands. Then launches a full attack on Sen'jin Village.