Unfortunatly HK is not Canada or Australia, and the practicalities of life here and roads are very different.
While it would be fantastic if you could book a taxi with a carseat, I for one would hate it if it was law and you had to wait hours for your taxi to pick you up, (as happens often in Australia) just for a 15 min ride.
It would get to the point where you just wouldn't go anywhere.
Of course we should use capsules and carseats for our babies and restraints for our older children when we can, but I suspect those of you that are so judgemental of others either have cars, drivers, nannies, helpers, all of the above or have never had to travel with more than one child at a time!
On the topic of taxi drivers driving fast and dangerously, if you don't like the way he drives ask him to slow down and if he doesn't get out, and change taxis.