Broken Encapsulation, how hacking is really just finding ways to play middle man to component systems.
Engineering systems is about learning to cleverly use abstractions and functional encapsulation to built extremely complex systems. In object oriented programming the concepts of encapsulation are a fundamental aspect of good OO design principles and I've written extensively on these ideas in posts in the past. Exploitation in the physical realm of hardware systems is realized by breaking the encapsulation of the physical components of the system (in this case a car) and then introducing a middle man to modulate the system directly or remotely. When one sees them this way it really is no surprise that they are possible, in OO programming the encapsulation layers are hidden behind layers of code abstractions that never present themselves for manipulation by outside agents. In software only the interface presented to the end user affords the possibility of an input vector for committing some exploit (for example cross site scripting and validation attacks) but these offer a very small attack surface and one that can be readily fixed by software engineers once discovered.
Hardware is a bit different, most of the components in your car or tv or computer are directly accessible by you simply with the aid of a screw driver in many cases, you can break through the layers of physical encapsulation that otherwise would mask the interfaces you are presented from your ability to change the deeper components that present the functions in those interfaces. So it is in this exploit where the hackers had to gain physical access to the various control computers in the car in order to hijack and modulate their inputs and outputs...this type of broken encapsulation affords an attack surface that is not fixed as it is in a software UI where access to chop down behind the interface is not provided outside of the functional buttons, fields, drop downs provided by the engineers.
The attack surface for physical devices can be expanded by third party access...the breaking of the contracts of encapsulation that enable component based systems of hardware to function creates vulnerabilities for modulation of the interaction of those components...this should make perfect sense and should not be seen as a flaw in the design of the systems. A car computer should not necessarily have to be more protected from external access than they currently are by the physical barriers of the car that keeps them hidden and to expect that car companies build them such that such systems are impervious to access can be done but will come quite literally at a higher cost.