I've taken to "tagging" notes in shared notebooks by simply including a line in the note body like Tags: tag1, tag2, ... This way I can at least search for tags, even if they don't show up in the left panel.
@engberg Is that the official response, or just a description of the current behavior? This seems like a major limitation to me. As a premium user, if I grant someone access to a notebook I expect them to make changes to it responsibly. After all, they clearly have the power to "pollute" my notebook with new notes and content. Tags should be editable by anyone with write access to the note. I consider any other behavior to be a bug.