Well, to play the devils advocate: How can anybody [youtube|copyright holder, etc] determine between what is Fair Use and what is one in a string of sixty 90-second clips that in total constitute the entirety* of a 1h30min movie disguised as fair use? Your 90-second clip is uploaded without context (ie. it's uploaded without reference to your blog, which is what really constitutes the Fair Use). And as other mentioned, it's only the clips that have been added by copyright holders that are protected (so it's not matching against infinity, but only those that wish to seek protection). *These clips exists in YouTube by the bucket, movies, tv-shows, you name it.