Pretty simple, from the RW7 rules:

Walkers that employ a rotary camshaft to actuate feet will not qualify for the weight advantage (shufflebots). No walker must employ any device other than moveable legs to support its weight. No type of rolling or sliding mechanism will be allowed.

Note it doesnt say the camshafts are the feet (they arent most of the time) but if it uses a rotary camshaft to actuate feet. so no matter how much more elaborate you make the system, rotary camshaft == no walker bonus.

This is also the main reason you dont see many walkers. The weight bonus doesnt justify the complexity of the mechanism. KISS is always better so roboteers will always prefer standard wheels. The double the weight if you make it walk may look enticing but its not really. After all, a rotary camshaft is no more than a wonky wheel.