That's true, but why does that mean they need to shut down her account just because she visited Cuba? Would that mean a grocery store would then have to refuse to sell you their goods because you visited the country once?
Your analogy about grocery stores doesn't really work because logging into Slack isn't the same thing as walking into a grocery store, because buying from a grocery store isn't the same thing as exporting food across a national boarder, and because neither food nor medicine is embargoed.
Yes, it harms their customers, but that harm and the resulting damages to Slack' reputation (and maybe legal costs), is what they must pay for being negligent in the past.