Food Subsidies and Taxes

There is an ad on TV around here that shows a lady complaining that the government is threatening to tax some of the foods that she buys for her family. Not fair, she proclaims. Well, guess what. It already happens in that some of the foods that we take for granted are that way through government subsidies. I'm wondering if this is the reason that junk food or snack food is often the cheapest items at the grocery store?