That always seemed bizarre to me. I get that printers, typewriters, and handwriting can all be identified, but why not just grab a common bic pen and hold it in your fist like a four year old, then print? I don't see how that could be matched to anything, and surely there's less room to screw up than when you're pasting out of a bunch of magazines and newspapers.
Agreed. I always figured this was a meme that made it easier for the police to track you down (just do some forensics on the magazine paper, fonts, etc., and locate someone with matching subscriptions).
"Hey boss, we got a subscriber to Mad Bomber Monthly, Torture World, and BYTE."
"iF YOu wAnT tO SEe yOUR deMOCRacY aGAIn lEAvE 5 miLLioN dOLlArs uNDer THE bRidGe AT ..."