Originally Posted by
carnarvon
It does not answer my question about buying a return ticket and not use it at all.
It does: you are entirely free to buy a return ticket and not use it all. This is buying a product and deciding not to use it.
What you are not free to do is to buy a return ticket from A to B and use it to go from A to C or from C to B, even if C happens to be an intermediate routing point in your itinerary from A to B.
This would be buying a product and substituting another product, which is not at all the same thing as buying a product and deciding not to use it.