They're not dictating what they eat. They're dictating what kind of food they get for free, on the gov't dime. That they CAN do. They want junk food? Fine, but you're going to buy it with your own money like everyone else does.
After I got fired from a job for refusing to do illegal work to an airplane that later crashed and killed 5 people, I was briefly on SNAP. I bought very very little "junk" food. In fact, I ate better on SNAP than I did OFF of it because then I could afford good wholesome quality food that I couldn't working for the hangar. I had just bought a brand new car and a house. I had very little left over for food. I was literally towing 20 million dollar aircraft out onto the ramp, and then hopping off the tug and eating a TV dinner for lunch. On a good day. Most days it was a cheese sandwich. It was nuts. One of the hangar owners pilots felt so bad for me, he told me "You single handedly run this place to perfection and you can't afford a decent lunch" most of the time when he came over to tinker with his jet, he would take me out to eat at Piccadilly. THAT WAS A TREAT.
So losing my job I was able to budget 100% SNAP into food. That helped a ton. 8 years of Obama ensured I didn't get paid well. Within 6 months of Trump starting his first term, my pay doubled.