Well the thing between them is "why are they considered touching and for what reason". If a ball on a rail is on that rail and is considered as a legal contact even though it does not need to move to be legal, then a cueball is also touching the other ball, and is also a legal contact. Why would one need to be hit back to the item it's touching but the other does not? If they both do, then that is pretty logical, if it's touching already, it is not a legal hit just because it's already there, it has to be moved to there. And if only one is OK but other is not, it's the same situation, both items frozen to some other one, but in one case it's a legal contact, in the other it's not, which breaks down logically.