My issue is that the phone is unlocked at that point and will take a number of seconds after the BP scan to re-lock itself. If some overzealous TSA employee who is either a BDO or playing BDO decides to "screen" (i.e., read and peruse) the electronic contents of my phone, I would rather it not be unlocked. If it's locked, it's no different than my laptop in that if they wish to examine the contents on the spot without confiscating the device (which they could not justify), they must have a password, which I will not provide. This sort of content-screening attempt is extremely rare but not unheard of--there are plenty of reports of TSA employees over zealously examining wallet contents, papers, and books, and there are a handful of reports of them over zealously examining a laptop computer.