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.