Sanctions work like this: One country decides "fuck that other country in particular", either alone by their power of sovereignty or together with some interna

Sanctions work like this: One country decides "fuck that other country in particular", either alone by their power of sovereignty or together with some international body. The sanctioning country (say, the US) now declares economic sanctions against another country (say, Iran). Since the US can only directly make rules for citizens/entities within the US, they say "it is illegal for anyone to export goods or services to anyone in Iran - if your company does it, the company gets fined and the CEO goes to prison". They might also say "and if anyone else that I can't punish directly sells to Iran, I will prohibit my people from selling things to you!" to force others to also participate in their sanctions. Now, the US gets a suspicion that Slack provided services to Iran. They arrange for the FBI (or whoever is responsible) to raid their offices, build a case beyond reasonable doubt that shows Slack provided services to a company in Iran, e.g. because said companies egress NAT IP was connecting and the user names match people working for that company in Iran. On top of that, they also show that Slack didn't do enough to prevent that from happening. Now, Slack gets fined and their CEO goes to jail (not sure if that's the penalty for sanction violations but I'd assume so). Since the CEO doesn't want to go to jail, and the company doesn't want to be fined, they'll do whatever they can to avoid selling services to Iran. They can generally choose who they do business with, and its in their best interest to err on the side of caution. You have little recourse if Slack doesn't do business with you because they don't like your IP, and they're almost certainly well covered by their ToS.