The zero sum assumption is based on "work" (as in human effort and human ingenuity) or "goods" being a fixed or certain amount. There is a certain amount of goods to go around, and everybody is entitled to some.
The point of "What one person receives without working for, another person must work for without receiving." is to criticize deliberate deficit in human effort. The grasshopper and the ant, if you will.
Okay. If the zero sum game assumption is incorrect (and I believe to a large extent it is incorrect in view of the fact this earth and sun are capable of producing more wealth than we can harness), may it not also be incorrect to assert that in every instance where one provides for another, he necessaily robs someone else?
BTW, far be it from me to argue in favor of government as a proper arbiter of wealth redistribution to the extent it rewards grasshoppers. It seems to me, however, both assertions place limits of a “zero sum” nature on what is available in terms of give and take.