Because moody, self-absorbed stalker-boys should not be allowed to get away with harassing and misleading teenage girls. "Buffy vs Edward (Twilight Remixed)" shows how everyone's favourite Slayer would put the dreadful Edward in his place.
(Via Dave and Dio on Facebook.)
Edited to add (July 9): Creator of the video mash-up Jonathan McIntosh discusses how and why he made the mash-up in "What Would Buffy Do? Notes on Dusting Edward Cullen".