If a Belgian citizen in Belgium visits your US server for commercial purposes then international trade treaties apply, and those treaties model your business (w

If a Belgian citizen in Belgium visits your US server for commercial purposes then international trade treaties apply, and those treaties model your business (which might just be a sole proprietorship) as having a Belgian subsidiary that is doing the actual commerce with them. Same reason that if a country Y has a law against selling thing X, but no law against buying thing X, then you, outside of country Y, are still not allowed to sell+ship X to people in country Y. For purposes of commercial interactions with people in country Y, you're acting as a local subsidiary subject to those laws. (In fact, for tax reasons, you may not even be able to sell into many countries without having a real established domestic incorporated business in those countries.) Note that this doesn't apply if there's a (multinational) import/export business involved — in which case, you have no obligation to avoid selling X into the country, because you're selling X to your own domestic country-Z arm of the importer/exporter. It's then the import/export business's duty to comply with laws about what can be sold in country Y (and to pay any import tariffs, etc.)