My understanding is that it's a membership organisation which exists to join together places which are keen to 'do the right thing' by their World Heritage Site status, and to work together to share information and support. I suspect the term 'city' is elastic, so that urban WHS can, if they wish, join, although technically not 'cities'.
Beemster:
http://en.wikipedia.org/wiki/BeemsterAs it costs cash to be a member, then it may be that some places don't feel they can afford the annual fees (strapped for cash councils are always seeking cuts, it's probably the same the world over). Non-membership possibly has more to do with bean-counting accountancy than desire of those responsible for heritage, which is frequently bottom of the funding and importance heap. It's also possible that certain WHS don't actually have the commitment and site management paperwork in place to feel as yet that they are quite up to scratch. Those that have have probably done so for sound reasons, connected with a belief in the aims of the organisation and its mission. It's possible others don't join as they disagree with certain of the aims, or don't have the management structures for their WHS which they feel could benefit. It may be that only a specific part of an urban centre is inscribed, and it's not considered a large enough part to be considered a World Heritage 'City', with specific management issues, eg the Statue of Liberty.
It's not mandatory to join, and I don't think it's altogether about making the most of WHS status, in terms of tourism (which although some places want WHS status because they see tourism benefits, is not really the heart of what World Heritage is about).
"The OWHC's MissionAn international non-profit non-governmental organization, the Organization of World Heritage Cities (OWHC) was created to assist member cities adapt and improve their management methods in relation to the specific requirements of having a site inscribed on the UNESCO World Heritage List.
In accordance with its General by Laws, the OWHC has the following goals:
contribute to implementation of the Convention concerning the Protection of the World Cultural and Natural Heritage and the International Charter for the Protection of Historic Cities;
encourage, on both regional and international levels, cooperation and the exchange of information and expertise among historic cities throughout the world in close collaboration with other organizations pursuing similar goals while emphasizing action likely to support the efforts of cities located in developing countries;
in cooperation with specialized organizations, ensure better links between research undertaken by specialists or experts and the needs of local management;
Sensitize the populations to patrimonial values and their protection"
It has various ways of achieving its mission, some of which are here:
http://www.ovpm.org/index.php?module=pagesetter&func=viewpub&tid=1&pid=57&mm=1039Paris is a member:
http://www.ovpm.org/en/france/paris