It's animation. There are absolutely no animations for any of the powers with walk. Same reason you can't use powers on the rocket board, void skiff, and carpet.
There are animations to go from neutral to walk and walk to neutral. There are animation to go from neutral to combat stance and combat stance to neutral. There are no animations to go from walk to combat stance. There are no animations to use powers outside of combat stance.
I don't remember the exact number, but one of the animators talked about it in detail once, about how many different animations shield needed added, and it was just a massive investment of time.
If they allowed powers to work with walk, they would need multiple new animations for every power in the game.
As things are now, I don't think they can add new animations. Every new power they add uses old animations, but with different auras. It works well, but it limits them.