Why do graduate student contracts state "RAs shall work no more than 20 hours per week?" I recently signed a graduate student contract.  I had outside funding for some time but recently became a normal research assistant (RA) again, so must have missed a contradictory clause when I signed a similar contract a few years ago.

I am a US citizen at a US university.

In the contract it explicitly states that graduate students "shall not work more than 20 hours per week on duties given by the advisor."

I have found similar language in other graduate student contracts at other universities, an example is here (scroll down to Duties and Time Commitments).  This example, however, states that RAs "shall not work on duties unrelated to research for more than 20 hours per week."  My contract seems to imply the opposite.

This certainly applies in the first 2-3 years of a Ph.D., since class loads typically require enough effort that RA's will not have enough time to work more than 20 hours per week on research.

But, after a graduate student has completed all class requirements, why is this clause still included?  

It is clearly contradictory, as I'm expected to work 40-50 hours per week on my research, especially when I have no more classes to take.  I have signed a legal document explicitly promising that I will not work the hours needed to finish my dissertation in a timely fashion.
If you are working at a US University, they often do this in order to ensure that you are ineligible for benefits under the family and medical  leave act (fmla).  If you work over 1250 hours they have to provide you with additional benefits, which cost more than they are willing to pay for your labor.

http://www.dol.gov/dol/topic/workhours/fmla.htm