That's a valid point as well. You can't create a movie about something you've never actually watched. But you need the incentive to create a spoof. Heres the formula:
Spoof= Hatred of a movie, book, character, global event, etc.
Creating a spoof= Equals understanding of the jokes and exploiting them.
To enjoy a spoof= Dislike of the theme, while understanding the jokes.
It's been said by the Vampires Suck people that Twilight heads are the ones who will most enjoy the movie, simply because they understand the series. Haters will go to see the movie based on the hatred of Twilight, but they won't understand half the jokes, unless they have watched the movies. Simple.