I don't like it
Sorry, it's a secretarial.
Why is he allowing secretarials if people are writing for the purpose having him sign?
Because hes a billionaire and celebrity and spends his time on his yatch with strippers instead of answering fan mail.
Others simply don't like to sign like David Gilmour, but wish to send something back to fans (in his case some kind of preprint)...the champagne and yachts and all that don't hurt ;) For many of these folks the sheer demand would have them signing constantly...