Ing tumbler

Ok kind of wondering why my Ing tumbler will slide out of the housing with the key in.Whats happening is when I shut the car off and pull the key out,i can pull the tumbler out with the key in it,not a big problem all I have to do is slide it back in,but what is so posed to keep it from coming out.69 Dart.I guess all someone would have to do is slide mine out and put theirs in and STEEL the car,now that's a big problem.