According to Australian Border control. They specifically swab feet and shoes, to test for traces of drugs on people they think look suspicious. Or people flying in from high risk areas.

And the times where I've flown to the US from Amsterdam, Munich, Frankfurt, and Copenhagen. I was always put in a room where they swabbed my stuff, hands, feet, phone, laptop (at the gate). And I asked them why and what they're looking for. They gave me the answer that they are mostly looking for drugs.

As drug smuggling is something they deal with every. Single. Day. where as people trying to board with explosives is extreamly rare.

I did not say they care if you have drugs in your system. I specifically said the opposite.

They just want to make sure you're not carrying drugs on you. Those who have drugs in their system, are far more likely to smuggle drugs than those who don't.