Because he accepted a blessing from Magatha. Just… don’t? I mean that’s not hard.
Also you’re applying 20/20 hindsite vision to Baine. Nobody knew she was in league with the Jailer until SLs and like it was said in some book, not sure which one:
I cant turn into a fart cloud like her, but even Sylvanas was scared of him
“Be mindful of your own words, Sylvanas,” Vol’jin said. “You don’t know when you gonna have to eat them.”
“Fortunately, I do not have to be mindful of what I say when all of Azeroth is watching, or else I might become as much a boot-licking Alliance sympathizer as—”
Baine did nothing so obvious as roar and seize her throat. He merely stopped in his tracks, gripped her upper arms, and squeezed. He was so gentle and precise in his movements and speech when off the battlefield that she had forgotten he was a warrior—and one of the finest the Horde could boast. He could, she realized belatedly, snap her arms like brittle twigs.
“I am not an Alliance sympathizer,” he said in a deep, calm voice. “Nor do I lick boots.”
“Let her go, Baine,” said Vol’jin, and Baine obeyed.