Same as, while install one software, most everybody keep clicking “next”, same on registering in a site - all just click “agree” and goes on. that because
- either they think all looks same and present same thing, even in allmost same words, so once one have read very few, will give up reading later;
- either they think they won`t break any rule;
- either they do not know that there may be important stuff.

Very good, Kuanhong, that you have writed about this subject, it should be more promoted.
Truth is that commenters in blogs are also bloggers and often they manage to stay decent.

Biggest problem is about copyrighting, while pics, texts, wehatever, all can be simply copy-pasted or, reff to blogs, taken thru rss readers (splogging).

Me, personaly, I am not at all a fan of keep in moderation firt-time-commenters, yet I watch very close activity on my pages (several times a day) while others may not want or may not have the chance ..
I treat comments depending on content of comment and not at all depending on who have writed .. probably this is one of reasons i`m not foreign affair minister of my country :-))