No, that's the rule.
The difference is subtle but there is one. If you have it on your knee pad then no matter how far back and forward your rifle is, it will always be supported well... however that isn't the same as if it's on your rifle, where the point of contact often does change depending on your position and what you shoot at.
Given that you're allowed to do it, ask yourself why it's not more popular...
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep