You are misinformed. It is not a rule unique to Delta. Delta also asks four qualifying questions when selecting an exit row seat online, one of which relates to extension use.

Southwest: http://www.southwest.com/html/travel...ency-exit.html

Alaska: http://www.alaskaair.com/content/tra...s-of-size.aspx

AirTran: http://www.airtran.com/help_seat_assignments.aspx