He has committed treason against the Horde on two separate occasions in favor of the Alliance, both of which were times that the Horde and Alliance were actively at war with one another.
Which included:
Baine warned Jaina ahead of time of Garrosh’s assault on Theramore, so she would have ample time to stage a defense.
And now he has gone out and actually killed Horde soldiers to set free an Alliance prison, and sent him back home.
Past that, there are other reasons that, when all added together, cause Baine to continue remaining a member of the Horde to not make any sense.
His father was slain in combat by the Horde’s fourth Warchief.
When the Grimtotem staged a coup and attempted to take over Thunder Bluff, it was the Alliance who sent aid, whereas the rest of the Horde told him to screw off.
His entire faction was kicked out of the Horde once.
And to top it all off, he’s penpals with the King of Stormwind, to the point where Baine sent a literal piece of himself to him.
Nowadays, all Baine does is complain about how “dishonorable the Horde has become”. Like dude, if you hate the Horde that much JUST LEAVE!