This is another example of software creating a natural monopoly. The car giants are right to have taken this defensive step because without them owning part of the supply chain, as soon as there is a clear leader in mapping or one of the other big data players pulls ahead, their ability to leverage their lead by having the best data will get them more customers which will create more data and so on...
The fact that the automobile manufacturers feel the need to defend against this shows how seriously they take the threat of "their" data falling into the wrong hands. And, indeed, the mapping operators recognise the value of "their" data as a strategic asset. However, the originators of the data (the consumers that pay for and drive the vehicles) aren't part of the conversation.
When Nokia put Here up for sale in April, the carmakers feared it could fall into the hands of a tech company, which could try to monopolise the data and cut them out of the race to provide new customer services.
http://www.ft.com/cms/s/0/3431f50a-39a7-11e5-8613-07d16aad2152.html#axzz3hjv2q46e