Of course, it never occurs to people that the bibles may have been the only material on hand to sign.
I have been donating food and caffeine drinks to the emergency workers in Alabama. Every time I go to the Emergency Center, I am asked to sign a poster for the survivors, wishing them prayers and good wishes. I was surprised because that never happened when I contributed stuff after 9/11. Obviously, an Alabama thing. It is a VERY religious state, God bless it.
Federal Rules of Evidence, Exceptions to Hearsay.
Rule 13: (13) Family Records. A statement of fact about personal or family history contained in a family record, such as a Bible, genealogy, chart, engraving on a ring, inscription on a portrait, or engraving on an urn or burial marker.
Apparently keeping records of personal or family history in a bible, presumably by writing on the inside cover, was such a common thing at one time, in some part of the US, that it was considered to be reliable information.