The force from the strap (to some extent) counters the force from the strings. The strings pull the pegbox in one direction, and the strap pulls it in more-or-less the opposite direction, so there is a cancellation effect.
A strap at the pegbox would be much worse if it pulled in the same direction as the strings. To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
