It doesn't need to. If it wasn't allowed it would be forbidden (in the same way that throwing forward is).
Because it isn't forbidden then it must be legal. The exceptions are covered as unsporting behaviour.
I tend to agree but was pooh-poohed in another thread for this sort of thinking.
OB said "If the laws were that good, then that approach would be fine. But they aren't.
We have:
(1) Technically illegal but accepted practice:
(a) throwing the ball forward for a kick
(b) scrum half picking the ball out of a clearly won ruck
(2) Not specifically outlawed, but not allowed
(a) spitting at an opponent
(b) yanking an opponent's pony tail"
This site is excellent for gaining an appreciation of the differing attitude that refs have. I'm not suggesting one is right and the other wrong. It's a tricky job and you should all be commended for the thought and consideration you give to the game (except Andy Scott)