It is a weird route. Casablanca is down the road, so you could almost conclude that Rabat and Casablanca (at least for long-haul) are somehow alternative (like
SAW or
IST). Yet when compared to Istanbul, Casablanca is a far smaller and less important market and a longer route!
However I wouldn't be surprised that the Moroccan government wants to develop RBA (they have built a new terminal) as it is the capital, so bilaterals are made in a way that more frequencies to RBA are allowed but not to
CMN. In this case Etihad would have no other way but to add new services to RBA.
Or that alternatively, maybe the Abu Dhabi government has interests in Rabat Airport itself. Emirati and Gulf companies have increased their investments in Morocco and the Magreb, replacing traditional French or European companies.
It would be interesting to know the reasons behind the route (instead of just adding more services to CMN), but I suspect there is something other than just commercial reasons.
[Edited 2015-11-15 07:27:59]