The "official" in OBCZ is sort of misleading. What it means is that a BookCrosser has made an arrangement with a business/shop owner/manager to help keep a spot (shelf, basket, window sill) stocked with books for clients/customers to have available. The officialness does not spring from http://www.bookcrossing.com
at all, except that recently, the site has taken to giving little icon/designations to the release zone and makring it as an OBCZ on the Go Hunting pages.
The kicker is that sometimes a book exchange shelf pre-exists BookCrossing involvement and sometimes it lives on after a BookCrosser stops maintaining it. Also, non-BookCrossers may drop off their own books for others to take and enjoy, but haven't registered them. So, the books wait until a kind BookCrosser can come and register them to BookCrossing (give them a BCID) and put them out again for others to find.
I have had a number of OBCZ zones that I've managed. A few of the owners insisted, even with BookCrossing signs, on the "take a book leave a book theory". I figured it was better to have BookCrossing a part of that than not there at all, so worked with them, within their rules. (Other owners have had other requirements: one says "No Danielle Steel", another said "No book covers with bouffant hairdos on women- go figure that one out.)
As my budget allows, I've left bookmarks etc there in the past. I do try and swing by the OBCZs every so often and label what's there, but one of my OBCZs is far away (actually in another state, but the shop owner loved the idea of BookCrossing and wanted books in her shop.) I get there maybe 6-7 times a year, but it survives without me, just fine. Sometimes it's full of unregistered books when I get there, sometimes it's totally bare.
As to the register/not register bit, I agree it's preferable if a book is registered, but not all people using the OBCZs are BookCrossers, so many will not bother. I'd rather them leave their books unregistered and take ours (and hopefully journal on them) than not participate at all. Any BookCrosser can register an unregistered book and help it start its' journey, so don't be shy!
You can also private message the manager of the OBCZ (FlyingStarOBCZ2, which is the Juan Tabo onen has its' own shelf and can receive PMS, as does FlyingStarOBCZ1, which is on Menaul) The other Flying Stars are not OBCZs though they may have bookshelves. You can also try PMing some of your active locals (jare and labmomnm and whiskeyjane come to mind) to see what's up.
Good luck and stay passionate about BookCrossing!