Some of the newer models actually do. I'm not sure if there are any which will override the drivers final decision, but that's probably not a great idea anyway.
I believe the way they currently work is that while the car in in cruise control, it is using some sensor to detect the distance to the car in front. It uses that distance to keep pace with traffic within certain limits. If they slam on the brakes, then your car does the same. Certainly the car will react faster than a human anyway, so in that case an override is moot. Since it's built as part of the cruise control, it can quickly be overridden by tapping the brakes.