Soaking up grain and corn syrup supplies is intentional. Ethanol in our gas has a similar purpose.
However, the primary reason you should not care about SNAP recipients spending money on soda or chips or junk is because it's usually a good price/calorie ratio, so for the half a percent of Americans that literally don't get enough to eat, it can be sustaining, if not healthy, but for the rest, the idea that people shouldn't be able to have a small luxury because it's socialized is just too much.
Taking candy from children is probably just not worth the squeeze. The entire federal SNAP program is ~$80 billion.
Lookup WIC. It is a very restricted program of food assistance, and spends immense effort and money of "only healthy" or "no junk" and parental education and supporting nutrition, and it really pays off, but it does that by relying on ENORMOUS free labor from parents and stores. A WIC checkout takes significantly longer than average, is more error prone, and is miserable for all involved, for like $30 of bread and cheese.