Unfortunately these mist nets do occur fairly regularly around Hong Kong agricultural land.
The justification I have heard previously is that they deter the birds from feeding on the crops, but I'm sure some will be taken for food and some kept as cagebirds.
As well as birds, fruit bats are regularly trapped and die in nets. Although it is mostly common species involved, I have seen Painted snipe and Japanese Sparrowhawk dead in mist nets before. What really troubles me is the knowledge that these birds and bats suffer a slow death struggling to escape from the net.
As John suggests, I would advise anyone finding an illegal mist net to report it to AFCD. They do seem to take this very seriously and will visit the site quickly to remove the nets. I think they usually just give a warning to the land owner that the nets are illegal and that they will inspect the site in future - this generally seems to be sufficient for most land owners to stop this illegal netting (I don't remember seeing any repeat offenders after I have reported the case to AFCD)