Self moderation doesn't always have to mean that you're censoring users. You could just be removing off topic, and unsubstantial posts, rather than relying on the forum moderators. That's a perfectly legitimate reason to open one up. You don't have to delete posts you don't agree with.
In the real sense of that expression, your first sentence as quoted should be the reason for having a self-moderated thread. Sadly, your last sentence is often why most users self moderate. It hurts when one finds one's post deleted for differing to OP's opinion. It suppresses healthy cross-fertilization of ideas when that happens . My observation is that despite the spam in the gambling board, moderators are hesitant in deleting posts there unlike their swift deletion when it comes to other boards. Do we take it to mean that moderators don't patrol that board often like they do others?
My suggestion for controlling spam on that board will be for moderators to lock any thread exceeding 50 pages, no matter how interesting anyone thinks the discussions going on there is. Secondly, campaigns should reduce their weekly post count requirement on that board to just five posts instead of 10. That way, participants won't hastily populate those threads with unconstructive posts and make them redundant.