The doors are to stop passengers who've already arrived landside, or meeters and greeters, or staff, from gaining entry to the Customs Hall, and from there to anywhere else airside. Two reasons: they might engage with passengers to defraud Customs, or they might smuggle banned substances/explosives/weapons airside as of course they won't have been x-rayed.
The "duty free" shop corridor is actually landside. The airside/landside boundary is somewhere between the Customs hall and that shop. MA just pretend it's airside to create the impression of "duty free" er "bargains".