You must have missed all of those articles on how low-income people buy junk food. :)
Even a low-income person has choices as to how that income is used. Do I pay for better food and do without cable? Buy used clothing so I can take the kids to a movie once a month? And so on... just because the definition of the poverty line uses a particular set of expenditures to determine the level doesn't mean that those in poverty will actually spend their money on exactly those things. As long as it fits in their budget, they are free to spend their money however they want, without a government nanny telling them that X is good and Y is bad.
Of course the poor have fewer choices of what to buy than the rich. But then again, this is pretty much the definition of poor vs. rich in the first place.
Setting a spending level is simple social engineering. I know, my wife and I do it as parents.
We give our kids an allowance for all their needs. It covers their lunches, their field trips, required spending as well as discretionary spending. We never say no to them spending it on whatever they want because we know that they don't have enough to do that. The amount we set is the minimum they need. It doesn't take a child more than one missed lunch to understand he doesn't have any real flexibility.