Besides the reasons Erin provided, it may be that they are justifiably concerned for the safety of their patrons (and personal liability

).
I can see the headlines now:
"Dead wheelchair user's family sues Carl's Jr for not providing bright enough lights in the drive through lane" . . .
Wheelchairs are not licensed and/or permitted on the streets either . . . for good reason. It is not because of discrimination, it is because they are no contest for a large 4X4, rig, or even small car. Many don't have running lights, and they certainly could not get out of the way in a hurry.
Dusk and dawn could be treacherous.
All the restaurants (here) have electric doors for wheelchair customers. I think it would just be safer to go to the inside order counter, like everyone else who is not in a licenced road vehicle.
Cherie
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I am not a Neurologist, Physician, Nurse, or Hairdresser ... but I have learned that it is not such a great idea to give oneself a haircut after three margaritas
.