Likewise, there is a difference between corruption and mind control
Kael, Illadin, Kil'jaden, and others are no more "courrpted" than Garrosh is. They all had desires and dreams and which they turned to dark powers to help them achieve them
People like Kael or Kil'jaden were not forced into joining the Legion, they both thought it was a dandy idea that would help with whatever they were trying to achieve
Something not like that would be Arthas when he picked up Frostmourne, as once he did that he became a slave to the will of the Lich King.