That's because in Japan a number of people actually work to death. Laeve work 11pm get up at 5am work on weekends.... Those people feel an obligation to do that, occasionally you get the odd person who dies from overwork.
So they have that statistic out of necessity. It's a common enough phenomenon.
I would say that reasoning is a little suspect. This is often considered a uniquely Japanese phenomenon, but there's a growing body of work suspecting that isn't the case.
In Japan part of the classification involves unlogged or extreme amounts of overtime, something we also have in the US (I have personally worked for years at companies that required a lot of unlogged/unpaid overtime from all of their employees). The cause of death is often stress related heart attack or stroke, again something we also have in the US. We know excessive job stress can cause permanent degenerative damage to the heart. So the fact that the Japanese choose to classify it as work-related and Americans don't doesn't necessarily mean such deaths don't exist in the US, or are necessarily too small to be worth measuring. They just simply aren't measured.
But they also respect people who nap at their desk! In high school I routinely slept in my first two classes (on one day it would be AP Physics, in the other it was AP Stats. I got a 4 in AP Physics). If I was in japanese work culture I would sleep at my desk all day, then do my 7 hours of work, and then shark the natives in Go from 11pm to 5am.
Though that doesn't seem to cover military/law enforcement or secondary deaths like suicide.
http://www.bls.gov/iif/oshcfoi1.htm