Transmission Slip Out Of Gear problems of the 2001 Ford Focus

Car was parked, transmission was in "park" keys out of car. A car backed into my parked car hitting the front bumper. My car was only 3 feet from the rear of the car that hit me and the backing car backed up slowly and only causing a small scrach on the bumper. After the 2 cars bumped, my car began to roll and continued to roll down a slow grade and picked up speed. The driver of the other car atempted to stop my car but was not able to get in sence the doors were locked. He also atempted to get the front tires to turn and could not. My car then struck a stand of trees distorying the rear end. My car rolled over 120 feet from the spot it was parked in. When I got into my car, I checked the shifter to see if it had been moved out of "park" it was still in "park" and I shook it to make sure it was. I then started the car with out any problem and moved it back to were it was parked. The car drove and acted fine, I even had 2 other people rock the car to see if we could get it to jump out of gear and it would not. I then called a Ford deal where I was told "thats not a problem, if your car gets hit it can jump out of park". Being a police officer my self for over 18 years and in that time with all the accdents I have see I have yet to see one where a car with a automatic transmission was hit and cause it to come out of "park" even when it was hit at over 50 miles an hour. My car was hit at less than 1 mile an hour and it jumps out of gear? but I really can't say it jumped out of gear sence it was still in park the hole time it was rolling down hill and was found in "park" after it came to a stop. This is a major safety problem and I thank god my car was not parked where it could of rolled out into a major intersection and cause the death of one of mine or someone elses family member.