Why did they set up a mini-TSA check point at my gate after we already went through the TSA check?

A couple of years ago at the Amsterdam airport, I bought a soft drink at one of the shops inside the terminal (after security). They put it in a duty-free sealed bag and told me to either drink it all before boarding or don’t open it until I got on the plane, otherwise I would need to throw it out at any secondary security screening.