The service booklet supplied with the pen need be neither filled in nor stamped by the dealer. The guarantee does not depend on it but on the proof of purchase: keep the receipt and you are just as well covered should the pen need attention. A stamp therefore brings the buyer no additional advantage.
For collectors, on the other hand, it is the untouched overall impression that counts. A blank booklet makes a set look as new and ties it to no particular dealer, no particular date and no particular first owner. On unused pieces passed on as mint, that increases demand and with it the price achievable. Conversely, a completed booklet can be useful, since it documents the year of purchase and the provenance, which is a help in judging authenticity, particularly on models that are frequently faked.
There is no official explanation from the manufacturer for this collectors' preference; it rests on market habit. Anyone who means to use his pen himself has nothing to lose from a dealer's stamp.