This doesn't really mean what you think it does. I run a fairly large internet business and spend half my waking hours analyzing and studying this kind of stuff, and I've been in the Alexa offices to discuss business opportunities. Although Alexa can and does track click behavior, the information you refer to is the result of their "editorial process", where they look for "similar" sites. When I check Alexa for one of our sites, they have a competitor's site listed that went out of business almost two years ago.
"Although Alexa can and does track click behavior, the information you refer to is the result of their "editorial process", where they look for "similar" sites."
You are suggesting that The Hollow Planet, the Billboard Liberation Front, and Mobilization for Global Justice are similar sites? Not buying it.
Plus, just a few weeks ago there were sites there that were more what one would expect, such as NRO and Frontpagemag and so forth.
Besides, Alexa's own explanation contradicts what you say is how it works:
For example, Alexa uses links on the crawled pages to find related sites. The day-to-day use of What's Related also helps build and refine the data. As the service is used, the requested URLs are logged. By looking at high-level trends, Netscape and Alexa can deduce relationships between web sites.