Is it? There is not one defining feature of autism which cannot be present in allistic people. There is not a single defining feature of autism that may be a risk factor to driving. The decision made is as arbitrary and baseless as expecting people of a certain hair colour, race or sexuality to notify DVLA or be at risk of £1000 fine.
How about the cost and the logistics of nearly a million autistic drivers (based on 2015 figures) being obliged to get their GPs say so on their driving ability in the way you have ? Is that worth worrying about? It takes 6 weeks to get an appointment at my GP already, 3-4 weeks just to get a blood test done. How much more pressure can GPs take?
How about the fact that countless medical conditions and physical problems that can affect driving do not require notification?
I could be deaf, blind in one eye, not be wearing spectacles I need to correct what vision I have because I couldn't be bothered to get my eyes tested, have a serious heart condition, ME, anger problems, BPD, arthritis that stops me turning my head, be suicidal & taking morphine daily for pain and they don't want to know unless in my opinion it affects my driving, but they threaten me with a fine if I don't tell them I'm autistic and get a doctor to sign me off?
Sounds like victimisation/discrimination to me.