Really, because that's what many people in this thread are advocating.Noone is saying government should rule people's lives. If you earn your own money you can buy whatever the hell you want. They're saying if you are on food stamps you should not be able to buy junk food and soda with it. Even if you completely discount all the health risks of living solely on junk food...that stuff is a TERRIBLE value for the money. The money spent on 2 bags of chips could provide several days worth of food if you bought stuff like beans, rice, and pasta.
Some people have pointed out that very occasional purchases of stuff like this is ok. That's fine. This can be accodomated. Say you can spend no more than x dollars per month on fast food type items. And it should be something small, like $5-$10.