My first inclination is that most of those people working more than 40 hours but making $0 in wages or salary are actually self-employed and thereby may not make there income as a salary or wage. Another large percentage of that group may be Unpaid Family Workers. You can identify both Self-Employed persons and Unpaid Family Workers with the detailed version of the variable CLASSWKR (classwkrd). Some other issues may be that people work on commission and either do not consider this a wage or salary or did not have a very productive year. There is no variable for identifying commission workers, but you can make a guess based on their Occupation, using the variable OCC1990. Along with salespeople, there seems to be clusters of Members of the Clergy and Farm Workers reporting $0 INCWAGE. These numbers are small, and you can think of easy ways to explain why they had $0 income but reported working. Also, it is important to recognize that these data are dependent upon actual responses, and there can be errors in reporting.
I hope this helps.