Probing the psychological motivations of people who contribute to open source projects would be a little bit heavy, but fortunately a majority of them seem to expect their reward to come in a form other than a cheap link ranking boost. Wordpress.org has pagerank 9 right now--it's going to be a target for this kind of posturing from people as long as its number is that high. If Woo themes had insisted their (PHP) menu system was proprietary, wouldn't that potentially violate the GPL anyway?