Sure, someone might want to read a protected question, comments, or answers to it. And someone might want to add a new comment.
But it would be helpful to know before visiting a question whether it is protected. That way, someone who might visit it to offer an answer or edit the question can know beforehand that new answers and edits are not allowed.
When a question has been determined to be a duplicate, you see [duplicate] in its title at the top level. And if migrated you see [migrated]. Why not similarly show [protected] for a protected question?