Remedy to keep flies out of the store?

Sign above till read, "Where the hell is Hamburg, California?" I ask, "why is that water bag hanging over the doorway like mistletoe?"

Store owner said, "It keeps the flies out." "About 90% of them will not pass the bag."

Klamath River area is the only place where I have seen this fly remedy in use. Tavern in Happy Camp had bag over its door also.