I don't think it's their culture. If that's true, why didn't they just walk right thro the door in a group. Why did they have to sneak out one-by-one?
It's just a very lame excuse.
1. Advertise that you can have abalone set dinner for $100 at your restaurant
2. Every customer who orders it would be given a plan congee only
3. If custome ...
That's a different situation. This is all-you-can-eat, so it doesn't specify how many dishes will come. As long as one dish came, you still have to pay the all-you-can-eat fee technically.
Besides, I think the article is referring to "you pay for what you have already eaten" not for stuff that is yet to arrive and unconsumed.
So in the case of your example, they can just pay for the congee and say bye bye
1. Advertise that you can have abalone set dinner for $100 at your restaurant
2. Every customer who orders it would be given a plan congee only
3. If customer complains, you would say that the abalone is coming
4. If customer dares to leave, sue away