I agree that the App market is not likely to be something that car makers care all that much about. While its likely great to have a bunch of fun driving apps available, the key revenue stream is the purchase of the vehicle with the head unit.
But that does not mean that the entire customer relationship is not worth protecting jealously. For example, certified auto parts are a huge business, as is the automotive aftermarket in general. The OEMs are very interested in blocking third party success in that market and keeping that revenue for themselves.
The OEMs have not been successful so far blocking software updates, there exists software to modify your ECU today. That software is significantly more prone to catastrophic failure because it bypasses things like the air to fuel mixture that the car maker carefully calibrated for safety and efficiency. Good article on this very topic: http://nyti.ms/Nf4Vry
I think allowing car owners to update their IVI systems would be much more appropriate and the argument currently being used by automotive legal eagles may not be particularly persuasive, i.e. blocking the GPL v3 is for safety's sake. I think there are a lot of business considerations that are behind this.