Anecdotal I know but from what I've seen they probably know they should put their backpacks in front, they just don't care. I went to a museum once where patrons were asked to either wear their backpacks in front of them or lock them up near the entrance. Literally almost everyone disregarded those instructions and put their backpacks on their back despite being asked directly multiple times to put their backpacks in front. They would for a quick second then move them to their back once the museum staff looked away. It was a real eye opener on people's self centeredness.