The port decanter should always be kept moving, hence there should be no need for it to be placed on the table. This has become corrupted into meaning that it 'must' not be so placed.
If someone fails to keep the decanter moving, such that it stays in front of him/her, the traditional query from diners to his/her left is "Do you know the Bishop of Norwich?" after the memory of said cleric who had a habit of getting so pickled that he would fall asleep before the port decanter arrived, creating a delay in accommodating the requirements of other diners.