I don't think it's reprehensible. Any user can from their settings select the option 'show dead' to see hellbanned comments/submissions. Often, you'll see people alert those hellbanned if they deserve to know. This is a free service open to the public with almost no effort required to post, so I understand why this type of moderation is done.
People who are used to seeing upvotes on their comments and submissions will notice when they have a long period of no interaction with the community and probably figure out they've been hellbanned. I'd guess most hellbanned users (who deserve it) find it comforting that their comments stopped getting downvotes and angry responses, so it might even be more humane moderation then letting the community rage at them.