I think the (ostensible) reason is similar to this, from the link: > As security expert Bruce Schneier likes to note, such screenings don't have to be perfect;

I think the (ostensible) reason is similar to this, from the link: > As security expert Bruce Schneier likes to note, such screenings don't have to be perfect; they just have to be good enough to make terrorists change their plans: "No terrorist is going to base his plot on getting a gun through airport security if there's a decent chance of getting caught, because the consequences of getting caught are too great." That is, the act of confiscating the liquids almost ensures that the liquids are already harmless. But if they didn't confiscate the liquid, it (supposedly) might not be harmless. It's a bit like Newcomb's Paradox: https://en.wikipedia.org/wiki/Newcomb%27s_paradox https://en.wikipedia.org/wiki/Newcomb%27s_paradox