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.

They are accused of smuggling synthetic marijuana - known by the street names K2 and Spice - into the country through the US military's postal service.

That might be a part of it

2.7 oz is a stupid amount of K2 (synthetic cannabis).

It's already a ton of natural cannabis. If you cup your hands together, that is about one ounce of weed which will keep you fucked up for a while. Now imagine 2.7 times that, and since it's synthetic cannabis it's way stronger.

Edit: To be clear I don't think weed should be illegal, and South Korea has draconic drug laws. I use it myself, which is how I know that 2.7 oz is a lot of weed.