According to Yen Sid, Sora and Riku, prior to 3D, were "self-taught keyblade wielders." So, in order to train them properly, he had them forget everything they knew about the keyblade and relearn it all. But, how does that make them any different from being self-taught? No one trains either Sora or Riku during 3D. They literally just relearn everything they knew about the keyblade (and more) on their own yet again. Yen Sid didn't even know where they were 98% of the game, so he certainly wasn't teaching them anything.