First of all, the word "loose" in this phrase is a synonym -- another word -- for "free". So "letting them loose" means "letting them free" or "letting them be free" because loose means not tied down or not held down to anything. "Turning them loose" just means "Setting them free", although colloquially we would just say in English "turning them loose" because it's just more common. Nobody would misunderstand you if you said "setting them loose" but it would not flow as smoothly since those words usually don't go together in the same phrase. "Turning them loose" "Setting them free" ^ That is how we would say either one in English. Hope this helps