The reason is that I've got a program called visidon which selectively locks the screen on an individual program basis and uses face unlock to unlock the tablet for personal programs. This is great for passing the tablet around with others and keeping my personal stuff private. The only hitch is that while I can face lock the settings screen, I can't stop it sharing some of the tablet contents when I plug it in via USB. If I could prevent the tablet from auto mounting when plugged in, then even if someone plugged the tablet into a computer they couldn't copy its contents. Admittedly, not much of the tablet's useful data is stored in a way that's Accessible even that day, but I'd like to try and tweak security where I can, and turning off auto mount means face unlock is required for viewing some tablet contents.