The moral of the story here is that holding multiple nationalities is a double edged sword - it can be very beneficial by eliminating visa costs and allowing longer stays etc., but it can also require special attention when traveling. This is why many countries recommend against dual-nationality even if they permit it. If the airlines tried to understand and enforce all the requirements it would no doubt just confuse matters more. Instead, anyone that wants to have dual-nationality should understand the implications of it, and the additional requirements on them.