It's actually something that the "powers that be" are actively working on changing. As I understand it, there are a number of factors that go into the rating. With open-submission platforms like this, the merits of all of the pages combined are considered in the ranking -- at least, before it moved to individual domains for each user, now it might be a little different. Hubpages also used to have no controls on the links used in articles and such aside from not repeating the same link too many times.
Over the past year or so, a lot of this has changed. We now have the individual domains, the "featured" and "not featured" statuses, the option to change links to "no follow" if they're untrustworthy, more editorial checks, etc. Changing the entire site's reputation will take time, but it seems like it's headed in the right direction as long as we as contributors keep doing our best to conform to the site expectations, keep publishing good work, and keep flagging poor quality or spammy hubs :).