I'm guessing you're referring to the rule regarding outbound links leading to one specific site. I've never asked "why", but it seems like a pretty logical rule to me, especially in its ability to hold down the spammers to a low roar. Without that, Sam Spammer could easily sign up here at HP, write a Hub, and point ten thousand links toward his unrelated (but now heavily backlinked) site.
Which would possibly do Sammy Boy a bit of commercial good (until Google's algorithm figured out he was an idiot, anyway), but would do NOTHING for HubPages.
It's a cool rule. I like it.