It's not surprising to me, it all ties back to the reality that it is more expensive to be poor than not. If you are poor, you most likely don't have the ability to keep a kitchen stocked with cooking essentials (spices are expensive), pots, pans, the increased water and heating cost to run a dishwasher and oven. Never mind the fact that most people living paycheck to paycheck are going from job to job and are physically exhausted at the end of the day. And that's for the people who were able to afford housing, then you have poor people stuck in shitty motels because they can't afford first month's rent and a security deposit. Those usually only have microwaves and a mini fridge.
Often times, going to McDonalds is your best option. The Government also subsidizes Big Food, so that factors. Getting 4 McDoubles for 4 dollars in 5 minutes driving through the drive thru next to your house so you can get back home to your kids and see them for the couple hours you have in between jobs versus spending 30 min to an hour buying a pound of ground beef, buns, cheese, lettuce, tomatos, the energy and utilities cost and the lost opportunity cost of the time spent cooking and cleaning when you could be working, it isn't hard to see why poor people choose the fast food.