On the same basis the USA bans GPL software, you aren't allowed to export stuff to naughty countries if you are American. The GPL doesn't allow you to block Cuba so nobody in the US is legally allowed to use GPL.

This doesn't seem to led to IBM, Oracle etc stopping selling Linux. Apple put a line in it's\ EULA about only being able to use app store for non-commercial use, which is ridiculous businessmen do use iPhones and iPads