So I've seen this in the wiki and also mentioned in the forums:

"In 5.0.x we removed the -i "Inherit" flag from folder grants and subfolder permission inheritance is now always TRUE. To prevent inheriting you must share the parent folder, then unshare each of the children by setting an explicit ACL on each of them. "

Zmmailbox - Zimbra :: Wiki

Which doesn't seem to have been correct for a few versions as I always see this behaviour:
mbox> selectmailbox
mbox> mfg / account rwidx
mbox> mfg /Calendar account none
ERROR: zclient.CLIENT_ERROR (unable to resolve zimbra id for:

Ie I was trying to share everything except the Calendar and it won't let me.

What am I doing wrong?

We could live with the shared calendar but unfortunately there's a real annoying behaviour in Zimbra that I can't figure out how to get around that if you have a Calendar in your list that has been shared with you like this all your appointments start coming from that user not your own.