Why does Hongkong Post levy the surcharge on the sender?

Sender has the responsibility to ensure the mail posted bears sufficient postage. Pursuant to section 21(4)(b) of the Post Office Ordinance (Cap. 98), if Hongkong Post is of the opinion that such postage or sum or deficiency should be paid by the sender and not by the addressee, Hongkong Post can levy the surcharge on the sender.