I think they mean that the Puritan work ethic is that working hard is good, always, and if you're not working hard, you're not being good, so they get on your case about doing something you like or something not traditionally seen as work (i.e. painting, writing novels, etc)
As for their mindless job, they might not see it as so. They could have stress and deadlines for their bullshit work and feel like they're working hard, even though the they are basically doing the same as taking a bucket out into the ocean and scooping up the water and dumping it out into the rest of the ocean.
So they're working hard and they're good, and you're just messing about, and you're bad. Which of course is not a healthy way of looking at things, hence the dark side of the Puritan work ethic.
