It's an obligation to the United Nations. The original intent was to make it possible for poorer countries to send mail to wealthy countries. Basically, members agreed that they would offer a service to deliver mail within their own borders for free/cheap so that sending mail in poorer countries would be affordable. It makes sense if the volume of mail is relatively low and symmetric, but it wasn't really intended for the sort of volumes Chinese discount sellers are using nowadays.

https://en.wikipedia.org/wiki/Universal_Postal_Union

There have been reforms in the works to help cover the costs in the terminal country, but I'm not sure if they have been effective or not.

http://news.upu.int/no_cache/nd/member-countries-adopt-new-t...

Alibaba also has some sort of logistics deal in place with the USPS to cut costs, but I can't really seem to find much in the way of details.

http://postalnews.com/blog/2015/09/16/chinas-alibaba-and-usp...