IMHO, it's more an american mindset, than anything else.
I don't think the customs forms are more complicate than in other countries, or that there are other major obstacles.
Quite the opposite, probably. It's just the image the average (read: not so informed) US citizen has of the outside world.
Every national culture is affected, up to a certain degree, by a nationalistic bias.
In the USA, the newspapers, the television, the popular culture, the way history and geography are studied, all that (and more) is very USA-centric.
It comes as no surprise that those, who are less aware of the outside world, think that out there is still Middle Age!
The sad thing is that it's a loss for both sides:
for the buyer, there are many items which are very dfficult to find outside the USA
for the seller, it means less profit, 90% of the times
Sent from my Android tablet
Yes, we are mostly hillbillies that only get to town/PO on Saturday evenings for the weekly picture show.