Treats are fine and it sounds like you are giving them treats correctly.I know so many members on here are "anti-treat". They consider everything that is not a true chicken feed a "treat". My flocks get a handful of scratch/mealworms/sunflower seeds in both the morning and evening. To me that is nothing different then they would get if they free ranged naturally. They would be scratching up worms, they would be eating seeds from various weeds and flowers, and if there was a cornfield near by you know they would be in that. I think there is more to the disease then just being diet based. Regardless, Betty's disease must have been hereditary because she wasn't even a year old yet. I know her breeder did not give "treats" and I never did because her and her flockmate would turn their nose up at the offer.
There is actually a treat guideline. Look at the table scraps guideline. It says to give them what they can eat in 10-15 minutes.
The problem is that some give too much and it cuts their nutrition. The hens then become un healthy.