While it's perfectly legal for your wife, a Canadian, to go to Cuba, it's still embargoed by the U.S., and Slack as a U.S. company must comply with the embargo (even though your wife has done nothing wrong per Canadian law).
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.
In this case, if the account was actually opened from Cuba ... that could be a problem.
What they should do is offer recourse and a way to have this resolved.
So they are absolutely offering recourse and resolution, but only where it is in their power to do so.
TLDR: Don’t expect Slack to be responsive to arguments that contain “please ignore US law for my individual circumstances”.