From the article:
Why aren't there Buffy The Vampire Slayer videos or Matrix music videos made by fans of the series?
WTF? There are. Just because you haven't seen them doesn't mean they don't exist. I've seen some Buffy videos, and I might make one if I run into a lull in my AMV making.
Grr. It's what Vlad said - the companies (like Fox) that hold the original series' copyrights beat down most sites that distribute them.
I'm not entirely buying the answers, but it is an interesting question, and I like seeing analyses from outside observers.