classical mechanics - Internal forces in open systems

We know, that the momentum of a closed system is conserved and thus all of the internal forces in such system must add up to zero. But in most mechanics textbooks it is stated, that even if there are some external forces acting on the system, the internal forces also balance each other and thus we arrive at a very usefull result, that the total rate of change of the systems momentum, is equal to the net external force acting on the system. How can we justify such an assumption about internal forces? Why they can not depend on this external influences?