Is there any web hosting regulation for foreign online retailers who want to ship products to US?

I'm a researcher consulting for a foreign e-commerce business. They want to start selling/ship their products to the USA through their website.

Apperantly, it is requiered that they have its website on a server in US in order to sell/ship products to US consumers.

I need your help to understand if this is really mandatory. It seems very strange to me that one MUST have servers in the US.