In real life, you have a greater peripheral vision which means in VR you have to turn your head and body even further to see behind. This is a real strain on my back in a dogfight. I think motion sickness in VR comes mainly from movements you are not in control of. If I press a button on my joystick to get a static view looking back, I'm in control of this action. You are expecting the reversed motion and it's only temporarily whilst holding the button. It feels fine when done in a racing game. I think you should still have the option to use all snap views in VR, I don't see why the game has disabled the ability to move the camera along this plane when it still allows movement along the X/Y plane. You don't have to use them if it does cause a sickness problem.