Are you sure about that? Do you know the tax laws of every single country in the world? Or even your own? Assuming the site is owned by a resident of the US, is it legal for a Cuban to use the site to do business with a US company?

It's not that it's impossible or insurmountable, just that it requires effort that a start up may not be willing or able to put in at release.

Exporting isn't as easy as selling in your native country. People are able to do it because someone went through the effort of studying the laws of the country being exported to and determined that it would be profitable to export to that country.