I could be completely wrong on this, but I think there's a bit of positive punishment at work.
If you do something quickly, and it is of high quality, it will probably get noticed, and your reward in addition to a sense of satisfaction, is more work. The satisfaction high won't override the punishment of more work, although salary will. However, salaried employees don't get paid more to do more work (bonus notwithstanding.)
Knowing that, it's not a far jump to realize that you can minimize your work but keep your salary, so long as your work output doesn't dip enough that you get fired.
If you do something quickly, and it is of high quality, it will probably get noticed, and your reward in addition to a sense of satisfaction, is more work. The satisfaction high won't override the punishment of more work, although salary will. However, salaried employees don't get paid more to do more work (bonus notwithstanding.)
Knowing that, it's not a far jump to realize that you can minimize your work but keep your salary, so long as your work output doesn't dip enough that you get fired.