When the boss says "someone pooped in the urinal", a McDonald's employee needs to say "yes sir" and go put on gloves. They also need to show up to work on time, take the 6AM shift when asked to, etc. Not all people are willing to do this.

According to many sociologists, a large number of lower class Americans lack conscientiousness. Go read this dead-tree book, for example (sorry, online sources are hard to find):

http://www.amazon.com/Promises-Can-Keep-Motherhood-Marriage/...

The focus of the book is on why poor single women choose to become single mothers, but it gives a great look into the work habits of the lower classes as well.