Because that's how embargoes work? They impact not only goods, but also services. The US has one on Iran. And as Slack is a US company (or rather, a company which has an interest of maintaining operations in the US, regardless of where it is based), it complies, thereby blocking service to any entities (e.g. people) in Iran.
Exactly. Embargos function by taking away the rights away of American businesses and people. Trade embargoes make it illegal for US entities and people to trade with certain groups of people.
All US companies doing business internationally are impacted by US trade laws (eg; sanctions, trade embargoes), I believe.
Eg; you're not allowed to sell stuff to Iran, or certain individuals.
They can also bend those restrictions to non-US companies wanting to do business within the US. Just because the US embargoes/sanctions the sell of Iranian oil, or the sell of any nation's oil to North Korea, doesn't mean that it is illegal for other nation's companies to do so (unless their gov't also has similar restrictions). Here comes the but... the US then makes it illegal for those companies to do business within the US just because they were friendly to people the US doesn't like.
Any US company is not allowed to do business/operate in embargoed countries. That's what an embargo means.
I'm puzzled. Does that mean nobody uses Windows in those countries? Same for Google/FB?