Having a sense of social justice does not automatically mean a person endorses all food programs for the poor.
There has been a thoughtful, fairly gentle, literature over the years, in which some anti-poverty advocates raise important concerns and questions about the food bank network that supplies the nation's charitable soup kitchens and food pantries. Perhaps the most notable reference is Janet Poppendieck's book, Sweet Charity.
A recent, provocative, contribution -- available without fee on the internet -- is a December article (.pdf) by Beth Osborne Daponte and Shannon Bade in the journal Nonprofit and Voluntary Sector Quarterly (in the blogosphere, see also this summary). Notwithstanding the private character of the charitable food donation system, Daponte and Bade attribute its rapid growth in recent years to public policy -- that is, changes in the government's commodity donation programs such as The Emergency Food Assistance Program (TEFAP) and changes in the FSP that leave participants in need of inexpensive supplementary food sources. About TEFAP, they write: "Compared to the FSP, TEFAP is administered inequitably and in an ad hoc manner."
Daponte and Bade discuss one sign that the private system is not entirely complementary to the Food Stamp Program: the surprisingly high rates of FSP non-participation by food pantry participants. Here, for example, is a graphic from an ERS article by Laura Tiehen in 2004. One can think of a number of ways in which the Food Stamp Program serves low-income Americans better -- it's larger in scale, it uses the strengths of the market economy better by employing normal retail channels, it is less patronizing, and it offers families greater choice over what to purchase.