Anyone who brings this up has absolutely no clue about the swearing-in process or their own government at all.
Why -- O why -- do they get so much airtime?? Is it the Fox network??
There is no Holy Book required at the swearing-in ceremony. You could swear in using the Fanny Farmer Cookbook...or Uncle Remus...or some Beatrix Potter book.
Ooooooo, but, God forbid you use a Holy Book like the Koran. Lordy, Lordy. Lordy! All HECK could break loose! Hang on, people...!