I remember in the 90s (around 96') we had an additional restriction on Minimum flight weight. If this weight could not be acheived by payload, then we needed to use fuel to bring the weight up to this limit. This additional fuel would be classed as unusable fuel and fuel calculations for the flight would not take this fuel into account.
This was when I worked with Fedex and the aircraft was an A310F. I understood this was due to the less desirable handling of a light aircraft.
Anilv