This is definitely a question, not a criticism or negative post.
As I mentioned earlier, my mumma hens use flies as a training aid for their babies but once they have grown up, flies are pretty much ignored. They might still catch the occasional one.
However, I have read that flies can lead to tapeworm and also spread bacteria including Salmonella, Campylobacter, E.Coli and others. What have the flies been feeding on before the chickens eat them? So, while catching the occasional fly is probably harmless, would having a smorgasbord of them available for chickens to eat in large quantities not increase those risks?