I know this is considered grim, but put it this way....
A single brood of 30 S. formosus juveniles is likely to earn a harvester $US300500, which is a substantial amount given that almost 80% of the Cambodian population lives on less than US$2 per day.
$500 is enough to live off for a year for a family in Cambodia. That's from a single brood. Cambodia is a VERY poor country, and if you can do this, to keep your daughters from prostituting, and live a decent life in your town, and in three months you can have a good amount of savings, why not? For them, given the alternative, I can see why they do it.
They could be harvesting opium like others...
AND in the article, it states how they see that it's bad for the fish, and most aren't harvesting that way.
However, in the last 510 years, many harvesters have become aware that killing adult fish will decrease future populations, and reported that harvesting methods that are lethal to adult fish are now less common.
Lastly, the article states that damming the rivers is going to cause more damage than harvesting will...
Thus is life, it's too bad things work out the way they do...