It's a completely different computer—compare it to your typical web browser/web server model. The media computer can request display the battery information (the browser can get a list of products) but it cannot hit the accelerator (the browser cannot read other users' personal information). Even if you have root on the media center (your laptop), that doesn't make it so you can run any and all commands on the computer that regulates steering and acceleration (the web server).

there is a gateway between the media center canbus and the critical canbus. the gateway buffers all the data from the critical canbus and rate limits any requests for data which isn't broadcast regularly on the critical canbus.

You would have to get UID=0 on the canbus gateway to make requests 'willy nilly' on the critical canbus. Having UID=0 on the media centre would only help in making willy nilly requests to the gateway.

edit: clarity

The chrysler was a completely different system, but it was indeed infecting the car from the radio / entertainment system. The tesla is completely different from that. They have one entire computer system that runs the "entertainment console". You can even reboot it while you are driving and nothing happens (this is by design of course). The driving controls are totally separated. The only thing you notice when you reboot that console if that the radio goes off, the display goes black during the reboot (10 seconds). Then it comes back and all is well. Press both turning cylinder controls on the steering wheel for a few seconds to start the reboot. They designed it very well. That doesn't mean it is perfect, but they have approached security correctly as far as I can tell from outside. They pay for exploits too - see https://bugcrowd.com/tesla. They have hired security researchers to work on their system and attack it.

As a tesla owner, I do wish they would hurry up and publish their app platform. They do have apps that they wrote themselves, that come with the car.

And I really wish they'd update their web browser, and even more wish they supported linux. Maybe the chromebook os support will be secure enough for android apps that even tesla could use it.