If the service was never set up, then there's nothing to cancel (other than a demo which I assume is easy to turn back on). If the service has already been set up, then they already have your personally identifying information and there is no problem reaffirming who you are.

For what it's worth, I've spent the last week disabling Car-Net on my car. The number of people who told me it's impossible and that it can't be done is a little upsetting. I really wanted to just manually disable it myself, but it seemed like I'd have to risk losing my warranty if I did so.

EDIT: To be clear, when I say disable, I mean it is impossible for Car-Net to contact my car or for my car to send out information wirelessly. I'd guess ~99% of dealers (I honestly don't think that is an exaggeration) have actually never performed this service before for anyone, and the vast majority are not even aware it is possible.

I think I saw this a little differently, the account the user was trying to cancel shouldn't have had any information attached to it anyway. So by OnStar asking for personal identification on an account that (probably) doesn't have any information to begin with seems a bit ridiculous. So to prevent fraud, I can imagine most users who actually use the account already have real information attached to it. So pushing for personal identification before removal makes much more sense in that case.