Jon, there are some country issues that have been handed off to the airlines as well. What I believe is the background for this overcautious approach, thus bouncing back into the lap of the airline:

If you are flown to a country that you are not eligible to enter (China is where I can refer to) the airline receives a huge fine.

excerpt from China travel guide:

In situations where there might be some doubt you should check with your airline in advance and if they accept your situation try to get that in writing. Each airline decides its own approach to this issue so check more than one before you purchase.

http://www.travelchinaguide.com/embassy/visa/revoke.htm