It is relevant. There's a material difference between shipping material overseas and shipping it (and handling it) within the destination country.

If someone mails $ProhibitedItem at a USPS to the UK, then it's the job of local UK police and/or customs to reject the parcel if it is prohibited. It's the UK's problem, de facto if not de jure, because the sender is out of reach.

If someone with a UK subsidiary and local processing center mails $ProhibitedItem to their center and delivers it to someone in the UK, then that's more than the UK's problem.

And on an electronic delivery, is a great firewall the equivalent of customs? And therfore the only way to enforce sovereignty?