is it out of habit?
is it out of a sense of misplaced courtesy and respect?
let's say somebody was reading a page of the book you were reading - you are sitting on a bus and this person is sitting on a raised seat behind you, so he/she is effectively reading your book over your shoulder - what would you do?
would you:
- be really annoyed and just close your book and do something else?
- not care and just continue reading?
- hold the page open a little longer while pretending to read and flip the page when you think your fellow reader has finished, even though you finished the pages quite some time ago?
... right?