Interestingly, they only do this for RUH, and not DOH, MCT etc. QR on the other hand, for example, runs its DXB-DOH as F (I think there's one w/b on that route that's a 3 class).

I suspect this is because of the high Saudi - UAE premium traffic that wants to travel in 'First' and doesn't care what value for money that represents. Also, because these 1 hour premium seats are only ever going to be booked by corporations, the fares can be fairly outrageous. DXB-MCT is usually AED 3500 in J!