My suspicion is that the motivation for tracking SNAP purchases is to provide justification for adding new restrictions on SNAP benefits. It is an idea that is popular with many on the left and the right. In a poll I conducted last year, almost 70% of US citizens support a policy that would "prohibit purchases of certain food, like sugared soda, with food stamp benefits." That same survey showed that over 80% of respondents favored a policy that would "provide funding so that food stamps can be used to purchase foods at farmers markets." There have been real world efforts to subsidize fruit and vegetable purchases when using SNAP (see this study for one analysis of the efficacy of such a proposal).
So, what we're seeing is a shift from seeing SNAP as a tool to help with food insecurity (or hunger) to one that attempts to promote certain dietary patterns, presumably to promote public health. Ironically, this comes about as news stories are revealing a doubling in the number of children receiving food assistance in recent years, and research showing that the prevalence of food insecurity in the US remains high.
I have mixed feelings about the efforts to scrutinize SNAP spending. On the one hand, it is a public program funded with tax dollars, and it seems taxpayers should expect some form of accountability. On the other hand, we don't know a whole lot about the costs and benefits of SNAP restrictions or whether there will be a trade off between hunger-fighting and healthy-promoting goals.
One of the things that bugs me is the lack of recognition that proposed SNAP restrictions are likely to be totally ineffectual. Take, for example, a policy that would ban using SNAP dollars to purchase soda. That policy might make us feel good, but it isn't likely to have any effect on the amount of soda people drink. Why? Because people can re-allocate their budget to achieve the same bundle of food regardless of whether the restriction is in place.
An example might help illustrate. Suppose you receive $130 in SNAP benefits each month (this is the about average monthly amount received per recipient person in the US), and you spend another $200 from your own pocket on food each month (for a total of $130 + $200 = $320). Now, let's suppose you take one big shopping trip to the grocery store each month, and your cart is piled up with food (including a case of Coke costing $10). You've put just enough food in the cart to consume your entire budget of $320. Now, you know that your SNAP benefits can't cover the entire amount. So, what do you do? You pull out that little plastic barrier, put it on the convener belt, and put $130 on one side (to be paid for with the SNAP benefits on the EBT card) and put $200 on the other side (to be paid for with your cash).
Now, let's suppose we have a ban on buying soda with SNAP. What happens? You simply pick up the $10 case of Coke that was on the SNAP side of the barrier and move it to the other side of the plastic barrier. But, now you've got an extra $10 you can spend in SNAP benefits (and now you're also short $10 cash). All you've got to do is find another item on the cash side of the barrier worth $10, pick it up and move to the SNAP side, and your done. The end result is the same regardless of whether the SNAP restriction is in place or not: you spend $320 and drink Coke. The only difference is where the Coke is put on the conveyor belt. This is an insight that's been known since at least the 1940s (see this paper by Herman Southworth), and yet it is one I rarely see mentioned discussion about attempts to improve health by restricting SNAP purchases.
What about policies that would subsidize fruit and vegetable consumption? First, we should recognize that this policy is unlikely to reduce prevalence of obesity. Second, note that this policy is akin to increasing funding for SNAP, but in a restrictive way. We're giving people more money, but with strings attached. We're being paternalistic, failing to respect how people want to spend these dollars. It reminds me of this article from the Onion with the heading, "Woman a leading authority on what shouldn't be in poor people's grocery carts."