According to several TSA agents in my travels to and around the US, at least, when I asked why my water had to be discarded, they all said variations of the same thing, "it could be an explosive". And the news broadcasts I've seen when this measure was first implemented were telling people that these "new types of explosives" look like water and are hidden inside water bottles, and the water can even be sipped on without harm to the person from the heavier-than-water liquid explosive. So, while it may have been a lie, it was one that approved the measures. "False positives" were never communicated.