Because the cell phone's time is generally perfect, and you can pull the time out of the clock from the car for some time after the accident. Modern clocks drift seconds per month at most. When you read the time of the airbag event or similar you can read the current time and compare to accurate time.

Our world is becoming much too connected to anonymize. Every bit of data is connected to others and it leads through chains to fully identified data. Had we wanted to we could have identified every single user in the AOL anonymized clickstreams, not just the few who searched for their own names.

Anyway, as I noted, the car's data isn't even needed. The cell phone company now knows where you are, how fast you are moving, and when you stop moving, should it dare to record that.