See (and maybe vote for):
Bug 6418 – IMAP: hide or limit access to additional contacts and calendar folders
see also:
Bug 6019 – expose contacts via IMAP
In short this is a deliberate feature of Zimbra, bumping into the limitations of some programs (such as Mac Mail but also iPhone) that don't let you specify which folders you want to "subscribe to" over IMAP or ActiveSync.