It's convention. Whilst it's the same, one is a standard way of discussing something, and one isn't.
Just like people don't add negative cold water to coffee, or say that boats negative sink in water. Convention. When you press the loud pedal you accelerate. When you press the brake pedal you decelerate. Not many people say 'he jumped on the brakes and got a huge boost of acceleration', do they.
A downforce is a force, downwards. Downforce is never up, even though negative downforce would be up. Lift is always up - you never say a car (or a plane) has negative lift, even though that would be down.
How simple do we have to make it. Nearly 50 posts further on, and you still sound like a complete thicko on this matter [small disclaimer there, resolving me of blame when you think I've called you a thicko in all areas!].