They were quick enough to take your motorbility car order and I suspect did so without first explaning their courtesy car policy if indeed there is one and if so ask to see a copy and ask why this was not highlighted when ordering.
I would hope the dealers dedicated motorbility salesman would have been trained in such matters
Personally I would contact the dealers manager who I suspect has made up this ad hoc rule without head office approval
My arguement is fair and reasonable and one to put to motorbility if the manager fails to listen.
Remember as an available alternative
All electric cars are also autos and once fully charged will surfice as a courtesy car for a few days without needing to be recharged (they drive the same in fact better than ice autos)