That 600 lb payload is likely the legal rating - which is a limit the government imposes on kei trucks rather than what the truck is actually capable of. A bit like the "gentleman's agreement" in the past to limit cars to 276 HP that was, in reality, a lie. [1]

Daihatsu sells an "agriculture special" model Hijet with a special suspension that can carry far than 600 lbs [2]. During harvesting season, it's common to see them hauling around full sacks of rice which are rated for more than twice the legal limit [3]. A lot of the trucks that get imported to the US are probably the agriculture models.

The penalties for exceeding the weight limit can actually be pretty severe. But it seems police turn a blind eye to farmers making short trips to the processing plant on public roads.

[1] https://carbuzz.com/features/jdm-gentlemans-agreement-japane...

[2] https://www.daihatsu.co.jp/lineup/truck/02_grade.htm?item=5

[3] https://store.shopping.yahoo.co.jp/truetools/tnk-kaba-1300rc...