Had to do the same thing in ATH recently. Had to transfer everything from my sturdy ziplock bag that I have been using everywhere I go to a flimsy ziplock bag that had ATH airport name written on it. After I passed security I transferred everything back into my original bag and ditched the ATH bag.

BTW, When I asked why I had to to make this time-consuming and frivolous transfer (actually I just asked "why?"), I was told that "it's the law".

I swear, there are people somewhere, locked up in rooms for hours at a time, tasked with coming up with ways to further annoy travelers for no particular good reason. And they laugh.