I see the same behavior under Zimbra 6.0.5.
To be precise:
Tags I create aren't visible to people who have mounted my Address Book. (Not a big surprise.)
Those people also can't tag items in my Address Book for their own use.
Bugzilla has a lot of entries regarding
tagging shares; some of them look like they might be relevant to this issue.
One problem, overall, is lack of clarity on whether tags themselves should be shared. I think the concept of "global" vs. "local" tags may be the way to go here. I.e., normally a tag in a shared folder should appear only to the user who created/applied the tag. But there should be some sort of facility at either the user or admin level to create/publish "global" tags which, when applied, appear in everyone's account.
Probably you'd want a mask to control who will see each "global tag" and a separate one for being allowed to apply/remove each "global tag".