It's worse than you describe
OnStar has granted themselves the right to collect this information “for any purpose, at any time, provided that following collection of such location and speed information identifiable to your Vehicle, it is shared only on an anonymized basis.”
And as we've seen, there's no such thing as anonymised GPS data: if your car is consistently parked at your home, it's pretty obvious where you live; and pretty easy to figure out who who are from that. This combination of pin-point accuracy, and their prospective customers (law enforcement, marketers, etc.) makes this data collection and resale very disturbing.