This is due to the effect of free-fall. Both the thief and load will fall down with the same  acceleration due to gravity and as there is no relative motion bw them so the load will considered in rest as compared to thief thatsy no force appiled on the thief.. due to zero acceleration between them.. 
1 5 1
When an body is freely falling down, the weight (or the force) exerted by it is zero. This is because in relative to the thief, the load is in rest. That's why we say that motion is always relative.