I made contact with an Etihad rep who had assisted me before with a query and asked for the issue to be escalated as appropriate. The response that came back from the pricing team is that it is not an error, but rather:-
this is the fare introduced by our pricing team in the market for AUH-MLE sector.
A couple things. This has nothing to do with the fare, as the taxes and fees are correct when you buy a flight on that routing with cash. Interesting also that they consider adding $150 to the reward carrier surcharge as a "fare" adjustment.
Also, it would appear that they are operating outside their own t&c
http://www.etihadguest.com/en/terms-and-conditions/
Section 8.3.2.2 states
GuestSeat Rewards booking is subject to the payment of a carrier charge. This will be calculated based on the number of sectors and class of travel - US$ 50 per sector when travelling in Economy Class, US$ 100 per sector in Business Class and US$ 150 per sector in First Class. Application of the carrier charge issubject to local legal restrictions.
There is no provision in the t&c for changing the carrier surcharge based on the route. (Likely, there is probably some sort of "we can do anything we want" clause)
Finally, one wonders why change the carrier surcharge for guest seats on a single route ?