In short, this change in policy creates a kink in the demand curve for fast food, as low income EBT recipient customers increase their demand, while demand by higher income customers does not change.
Consider two fast food buying customers. Person A earns an income that disqualifies him from EBT. Person B earns a low enough income to qualify for EBT, and has chosen to receive those benefits.
Now consider Person B's more modestly budgeted demand curve. This is person B's demand for fast food meals before the change in policy that allows him to purchase fast food with EBT.
Notice how at a price of $12 per meal Person B will not buy any fast food meals. This is beyond his budget at this point. So what happens to Person B's demand curve when suddenly he can use EBT to purchase these meals? This change in policy will shift his demand curve for fast food to the right, because he will now be more able to purchase fast food meals at various prices. Here's his new demand curve:
We have now seen the effects of this change in policy on two individuals, one on food stamps and one not. But what about the market demand curve? And what does this mean for market equilibrium (the point where the quantity demanded equals the quantity supplied)? Let's find out. Because the market demand curve is the summation of all the individual demand curves, just imagine adding together all the demand curves for all the Person A-s and Person B-s of the market. Let's say the demand and supply curves before the policy change look like this:
The market price is at P1 and the quantity sold is at Q1. Suddenly the county government for this market allows EBT to be used to buy fast food. What would this do to the demand curve? Because higher income, non-food-stamp recipient customers like person A can affect the entire market demand curve, from the highest prices to the lowest, and lower income food-stamp recipient customers like person B would tend to only affect the lower parts of the demand curve, when EBT is suddenly allowed for fast food purchases, it is only the lower parts of the demand curve that will shift outward (in reality the change would probably not be this prominent but I have made it prominent just for demonstration). Here is the new market demand curve:
The result is a kink in the demand curve pushing out at the point where most food stamp recipients would be priced out of the market. Assuming the supply curve is below that point, this increases the equilibrium price to P2 and the equilibrium quantity to Q2. Thus this policy is a good thing for fast food companies, increasing their revenue by the amount of: