The engineers who designed the Predator were not idiots, adopting Security through Obscurity. The feeds were not encrypted for at least two reasons: The Predator is supposed to be able to go at a moment's notice, and having to wait around to be keyed for the mission at hand as required by NSA, defeats the purpose. I've heard the expression "80% of my intelligence needs that I can have NOW, can share it with coalitions, and don't need a security officer and a safe to transport it around the battlefield is a dream come true..." In any case, the imagery doesn't tell anyone anything they don't already know, and has a quick "half-life" meaning it rapidly becomes irrelevant. It's just not worth encrypting.
Not saying that we don't do stupid things, just that not encrypting the feed was not one of them.