Even if people did enable self-moderation on their topics, what makes you think that they would actually contribute to controlling spam? After all, controlling spam takes time, and few people like working for free. Moderators get paid to do their job, why would people do it for free? I understand it's for making Bitcointalk a better place for everybody, but usually as soon as that requires conscious individual effort, that pipedream goes out the window.
I agree that people are unlikely to want to perform such work, guided only by enthusiasm. They`ll prefer to see the results of their activities, and get encouragement for their work. To make the forum better is a noble goal, and many folks already do it by writing quality and useful posts, identifying spammers and giving practical recommendations. However, the result of such activities isn`t visible immediately. Very few people have the patience to work for the distant future. Therefore, I believe that attempts to improve the forum shouldn`t be chaotic, but systematized, "legislatively" formalized and directed by the administrator.