Actually, how I remember it, the LAS-NRT segment could also be included in the routing. At least that was the result of a fellow FTer's inquiry.
Why should it be forbidden to fly ZRH-LAS-NRT to get to Tokyo (instead of ZRH-NRT) and then have your return flight NRT-ZRH ? If you chose a routing via a different zone, they just charge you for the more expensive zone.

As to your question: since ZRH is not voluntary (apparently you have to transfer there), I don't think they can hang you for visiting Zurich twice on your trip.