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When it comes to pagerank, I'm surprised if Google hasn't discovered these links ages ago, and discussed if they should do some tweaking to avoid boosting the linked pages too much. You have to understand that pagerank is something that the majority of the ODP community don't care about. The sites are linked because they are concidered to be sister sites of the ODP, and because the staff and senior editors at MusicMoz and Open-Site are dedicated ODP editors as well. If you edit in the ODP for a few years and then start a non-profit directory running the same model you will probably get a link as well. Not that 850.000 links from one site matters more than a single link from another site.
If you look at [dmoz.org...] you see another level of linking - the icons that lead to relevant categories at the other directories.