Just read something about Black Hat Seo in a forum and checked google for competitive keywords like 'Buy Viagra' if some thing spammy is there. Plz go to http://www.google.com/search?q=buy+viagra&hl=en&rlz=1B3GGGL_enIN216IN216&start=0&sa=N and give alook at the result. .edu sites are selling viagra on top 20 listing of google SERP. How they do this? I mean the black hat guys place their webpage on the server of these sites?
Alot of the pages have been taken down now, I wouldn't be suprised if the sites had been contacted by Google.
I think its because spammers posting comments/threads on forums and blogs with .edu domains which the students and staffs use. My blog gets lots of trackbacks with Viagra words and a .edu link. That is why those sites restricts normal visitors to participate in their blogs/forums.
The reason spammers target .edu sites is because they can rank #1 for almost ANYTHING with just 1000 links. Even wikipedia and authority sites that have been around for 10 years with 20,000 links can be easily outranked with an .edu.
It kills me when people say Google is the best search engine. Why? Well, just because. This is something Google definitely needs to fix. Whatever scheme they develop to beat scammers, will eventually be circumvented. How does an established site appear lower in the rankings just because another site has an .edu. I guess they figure, "no one would ever spam an .edu."