How did Slack figure out this guy's ethnicity / citizenship / status? Why are they allowed to access that information and unilaterally act on it? Sounds like there's a valid question being asked here to me.
Slack is protecting their multi-billion dollar business and their investors which are both based in the U.S. If they do not, they could run into trouble and put far more at risk. This does not only affect Iran, it affects other countries too which the U.S. has active sanctions against.
Now, I admit that visiting the countries on the list aren't on my to-do-list, but if for example I decided to make a vacation trip to Cuba I really shouldn't lose my account at any US-based service just because of that.
So you would make people suffer and think that they would blame their own government for that and not the US?
Don't you think that makes you and your country evil?