This was because studies had regularly shown that doing so produced feelings of motion sickness. You could have teleportation or slow acceleration, but not any sudden movements like you would see in console FPS games.
I recently tried a couple of games on the Oculus and was surprised to see how common FPS-style 'walking' is in VR.
It make me pretty sick so I'm wondering: is there new research that refutes the 'golden rule', or did developers just decide to stop following it for some reason?