Adsense is reserved for Gold members only because that is Google's criterion. Google grants a revenue earning programs i.e adsense for only those who have good content writing skills.
It is usually supposed that a member who has reached a Gold level has passed atleast the test of being a content writer in the eyes of the editors of the respective website. Hence Google lets them apply for adsense.
Google then approves the account only for those who have written good articles. It evaluates the articles on certain rules and criteria like good grammar and semantics, useful content, writing skills, SEO techniques etc. Only when the member's articles pass through Google selection criteria they grant them the permission to earn revenue from their articles.
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Webmaster - MPSpider.com