Originally Posted by csycsycsy

Hi percysmith, brunos,

Perhaps OP's only been to the UK, with both UK US not having visible exit controls

But back to HKIA, all they need is a one way door/path that allows pax in departures to rejoin the arrivals stream and then let immigration decide whether the pax can be landed (OP is not trying to enter HK twice).
OP is allowed to use CX F's lounges, they can spend the whole time at CX's dime (not cheap), or CX can help OP 'get ride of themselves'. There is no extra immigration staff work than if OP just entered HK directly, only the security scan when first entering departures is 'wasted', balanced with how much economic spend OP might do in HK, and even if not, a degree of tourism good-will.
If OP was a HKie, I can understand airport 'forcing' them to cancel the ticket, but not for an international visitor.