Because without the comma it looks like they used the nuclear weapons on the drones. Which I'm sure will work very well but I'm glad they didn't. When you spea

Because without the comma it looks like they used the nuclear weapons on the drones. Which I'm sure will work very well but I'm glad they didn't. When you speak you can solve this with intonation, you don't need a pause. But in written language that aspect is lost so you do need it. Different wording could also have been used. Like "near base storing US nuclear weapons"