They like you to dress smartly just in case they upgrade you.
I tried getting my wife back earlier this year. The journey to HK was ok but why oh why do the HK ID staff try and treat you like dirt? It was the first time I'd done this, we were courteous but when asking questions like, "Where would you like us to queue?" or, "What do these pluses and minuses mean," we were either cut dead, ignored or dealt with in a manner in which it was obvious they were not happy in their work.
Why do some staff take the opportunity to treat their colleagues in such a manner? Air France in my experience are the complete opposite. It's as if the HK mentality is that if you are placed in a position to help an equal, you are serving them and, by definition, are therefore somehow lower than them.
Eloquence fails me; I need more sleep.
BB