The average car in the USA is 8 years old. Who is still using the same entertainment devices for 8 years in the home? People get a new CD/DVD/Blu-Ray player, whatever. The only option for the car is aftermarket devices, which don't integrate well with on-wheel controls and which the dealer doesn't get a piece of (they way they do for high-margin options like in-car entertainment.)
What the dealers need is an in-car platform for selling upgraded entertainment devices that locks out the aftermarket. So you wire in some safety-critical systems to the same network, then you have an excuse to make it dealer-only.