I would say it is not a scam. I guess it would have been easier to ask the owners why they felt tehy needed to have tehm .As that would have given the best answer possible,
The awd is not constant as i am sure you know and like most systems it only comes on when needed for the rear wheels if there is enough slip or loss of traction ,that it needs to be used.
Personally i would have the awd which is much better than a 2 wd if needed which is highly unlikely in an urban enviorment.