From the full article

'In South Korea, those convicted of trafficking marijuana face from five years to life in prison. Drug possession carries a maximum sentence of five years in prison or a fine of about 50m won ($37,200; £30,300).'

So even possession is punishable. They aren't going to care that it's a small amout, and with the cash they also confiscated they will assume there was more that had already been sold. That is where the trafficking comes in I would guess.