Assuming for the sake of argument that I'm not supposed to collaborate with someone in some particular place... why again do the authors of these clients care about that? Are the black-bag squad going to disappear them because they didn't warn us to be good? Actually the charitable interpretation is probably that this is a sort of cargo-culted license without much informed decision-making behind it.

Embargoes and sanctions have no ethical force behind them anyway. They are merely a way to harm poor people while ginning up more pretexts for more wars. Anyone who can oppose such monstrosities using git should do so.