OnStar has been able to unlock your car remotely for 15 years... this looks like a creative use of that system. If you disagree, you should manually disconnect the OnStar system.
Up until ~2009MY cars, if you didn't pay for OnStar, they never activated your vehicle and they would never have access.
I think it was mostly a liability / strategy decision rather than a cost one, but you can imagine people calling up and begging OnStar to start a subscription when they need it and then cancelling immediately after. Or trying to subscribe if their car was stolen to track it down..
