Not only does it violate assumptions, but searching a list for the absence of an item is not as reliable as finding an item on a list. You can positively confirm when you find something, but if you don't find the thing, it takes longer to confirm its absence because you have to check alternate hypotheses, like "is it perhaps listed under another name?" "did I scroll past it too quickly?" "is it my dyslexia flaring up again?" etc.
A useful place for this information would be on the itinerary where the seat information and check-in times are printed. Sure, a lot of people may skim this but a bold notation "online check-in is NOT available from this airport" would at least be a diligent and potentially helpful gesture on the part of the airline. And therefore it will never happen on AC.