Stricter restrictions to SNAP programs isn't a new idea, but one of the biggest things holding it back is just that it would be a mess to implement. Even if we accepted that there is a such thing as a standard healthy diet (which is debatable), what constitutes a healthy food? The amount of fat in a single serving? The amount of carbs or salt? Avocados and nut products can have more fat than a lot of junk foods. Some cuts of meat are more than 60% fat. To make bread, some sugar is required. So what separates a healthy food from junk food? New food is introduced all the time, and all of that shit would have to be reviewed for whether or not it would be eligible for purchase or not.
On top of that is the matter of accessibility. Food insecurity is often linked with homelessness, or at least domestic instability. For benefits recipients, buying processed food isn't strictly a matter of buying the gnarliest fast food they can get their hands on, it's often the result of not even having the resources to prepare a home-cooked meal. Some don't have kitchens or pantries, which would prevent them from storing and preparing food. For them, the only option is to buy food that can be eaten as it is bought or that requires minimal preparation.
Additionally, the reality is that "junk food" is often the wisest choice for people with fixed food budgets. While processed food doesn't usually fit anyone's definition of healthy, it's often extremely calorie dense for the price, shelf/freezer stable, and more readily available in areas with higher benefits usage. And a big part of why they are so affordable is because of the insane subsidies these industries get.
In general, I'm not on the same page as the rest of GB that food insecurity programs should be as punitive as possible.