Many of the other car manufacturers provide remote access to cars. See GM's OnStar, Ford's Sync Services, etc... One of those companies used to advertise on TV you calling them and having them unlock your car remotely and IIRC starting the engine.
I doubt they're really opt-in, in the face of a hack. Having your car still connect to the network means that they can activate a new account instantaneously and without user intervention. It means they could potentially sell non-subscribers on a car unlock right on the spot, rather than saying, "tough luck, you should have subscribed to our service, after you get a locksmith to unlock your door, call us back and we can help stop it from happening again."
I would bet five Zimbabwe dollars that most systems of this type can be used to control the cars of non-subscribers if taken over.
The app server was Thin last I checked. It's a Ruby app, though probably not Rails (hopefully not, at least). The streaming stuff is a node.js server. I assume they have a load balancer in front of it all.
Keep in mind this isn't for the telematics, which are an entirely different system. This just gives you a view into some parts of that system. This is primarily used by the mobile apps, so the activity level is likely much lower than that.
Also, which server are they using? That alone is an interesting fact, as it has to support tens of thousands of simultaneous connections...