Looking at it from a theoretical standpoint, the metaphysical side of the Laws of Magic is the effects of black magic warping the human using it. We can debate if it warps his mind, his soul, or his magic, but it warps the human using it.
Once a WCV has surrendered (or formed an alliance) with his demon, he's effectively no longer human. He has a hybrid demon/human mind, soul, and magic. Taking about the Laws of Magic and non-humans makes as much sense as comparing boat design to airplane design - there are some similarities (you have to take resist into account) but there are more differences (air resistance vs water resistance, lift vs buoyancy, etc).
If I'm right in this in, the Thomas could get the Law Breaker stunts the vast majority of his family can't.
Why do I think that the laws only apply to humans? Because fairies break them all the time.
Of course this is just my opinion, but it makes sense to me.
Richard