Actually I have proof it was on Sylvanas’s command.
“Baine is wary of us freeing him, as Sylvanas was expecting us and we’re falling into her trap. Sunreavers surround us, seeking revenge on Jaina for the Purge of Dalaran. However, we are able to escape.”
Magister Hathorel: Ah. Right on schedule as Sylvanas knew you would be.
Jaina: We do not wish to end any more lives. Let us take Baine and go.
Magister Hathorel: No, Traitor. Lady Proudmoore. Do you remember me?
Magister Hathorel: I am Magister Hathorel, one of the many innocent Sunreaver mages you sicced your Silver Covenant dogs upon.
Jaina: There was more violence than I had wished, but it was the Sunreavers who betrayed us, first.
Magister Hathorel: A few who were following the orders of their lawful Warchief. As I do now.
Looks like you were wrong here.