A 'tax ruling' is just another name for law, right?
Without getting into splitting hairs, no, not necessarily. For instance, a ruling from the Canada Revenue Agency about how a software sale would have to be accounted for buy Apple and how it would affect corporate taxes would not be an explicit law and would explain why no one can provide a pointer to such a law since such a ruling would likely be an internal procedural matter and hence not published in any easily-accessible venue. That's sort of what little information has come out is pointing to, but again I don't know for sure.. Apple clearly feels they have reasons for the policy, though; if it were just being difficult, they'd have the same policy everywhere, so
something in Canadian law, tax regulations, etc. is driving it. But to us end users, the reason really doesn't matter.
They could at least they refund the money for customers who were expecting to buy apps..
I would agree that if the restriction isn't printed on the gift card itself or otherwise made clear at the point of purchase, a refund would be in order. But Apple doesn't ask my opinion, and since they put the restriction in the iTunes Store terms of sale (to which we all technically agree when we create our accounts, though few people ever read the agreement), I think such a refund is highly unlikely.