Perhaps because the memory of 1982 (and of the great press-led downsizing scare of the early 1990s) is still strong? From the perspective of the functioning of the system as a whole, unemployment *is* a worker discipline device--a way of keeping workers afraid of the consequences of not working hard, or asking for higher wages, or whatever.
Even when unemployment is low, the fear that you may be fired and that unemployment may rise and that you 'll have a hard time getting a job (or getting a good job) may still be a powerful motivator among those sections of the labor force that have to be disciplined.
And as long as inflation stays low, productivity stays high, and the memory of 1982 fresh, the Federal Reserve Governors can continue to give each other high-fives and wonder if they shouldn't be willing to let measured unemployment drift down to 4%...
Brad DeLong (who has been teaching efficiency wage theories to the second-year graduate students)