I kinda feel cars are core to Apple. Essentially they are a screen company if you think about it. Their job is to own all your screens and make a quality experience across them, both the bonus of upselling services to said screens. A car dash is the last (others?) of the more used screens they haven't covered yet. So probably a market they dont want to miss out on, especially as it ties in with other areas so well like music, maps and in the future apps.