They should never have allowed the account to be created from Cuba in the first place. Slack when it was younger, didn't have good policies in place to actually follow US law. As such, now that they are reviewing their old records they realized they committed illegal actions that they need to clean up.

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.