Government subsidies are also responsible for a large part of the food production infrastructure -- not only is one of the biggest problems behind food transporting it from production to market, which is completely government subsidized (if food producers had to pay for the highways their trucks use/erode they would go out of business and not serve half the country) food not only is subsidized now, but HAS BEEN subsidized for every moment the industry has existed because it is a necessity. It is impossible to separate the food industry from government interference, they are permanently symbiotically joined. If food production can even now survive now without the government (it cannot in anything near its current form), this does not mean the progress we have today could have been made without it; it just means the government has done a good job of fostering and supporting the industry.

Food is heavily regulated, including the food bought by stamps. The government decides what can be purchased by the stamps, and every item of food sold in any store across the country is approved for safety and health. Without these regulations we would have massive constant food related deaths as producers compete on price and compromise safety to the detriment and possibly death of any customer who cannot afford the high quality "market regulated" product which is exactly what has happened in every unregulated industry in the history of the world. And for higher end products: the calculus of a PR cover up operation and possible civil suit vs. actually safely creating food is done by companies, and currently federal regulators weigh in on the side of 'you had better make this safe or else'. You are proposing pushing the balance here towards "well, if we can save a buck, screw it, we can tie the victims up in court until they go bankrupt and die anyway or blame it on the supply chain and promise we will do better" which makes safety much less important.

A health care spending card is perhaps a natural suggestion but is made all the more insidious because it is so. It is, in reality, an atrocious and murderous idea; the entire crux of health care is not only that it is essential to living and therefore has an infinite price and that quality is almost impossible for consumers to accurately assess but that needs for different people are drastically different, usually for reasons that have very little to do with their choices or desires, which is why it is generally taken care of by insurance while people do not buy "food insurance". Health care spending cards (or HSAs) are essentially a euphemism for condemning any poor person who gets sick to death.

And of course it makes a difference whether poor people can steal the food. Remember that unfettered free market capitalism is the system where a rich man's dog eats 5 course gourmet meals while his poor neighbor's child dies of hunger. Putting something necessary for survival behind an arbitrary chalked in line and saying "sorry, you don't get to have that because you weren't born rich and the neoliberal market economy has transitioned and has no room for your skillset, enjoy your one free death, maybe when you are reincarnated your father will be named Koch" is a very fast way to start riots and anarchy. The only reason the rich are rich is because the poor believe they are and act accordingly, we call this belief structure market society and government; shattering that necessary illusion will likely hurt everyone, but it most definitely hurts the rich. The point is that if food is available but inaccessible to the poor the free market and governmental structure in place will cease to exist -- the market and government have evolved in a very careful way to prevent this from happening.