Most cases it simply comes down to CYA from a FFL holder who just does not know the rules. I accept them and always have.

Having worked in a retail environment I do understand why some shop keepers decide to not honor them. I once received a Henry .22lr lever gun from an individual, magazine fully loaded and one in the chamber.

http://www.bertramandco.com/
Booking African hunts, firearms import services

Here for the meltdowns