The ADXL78 series [1] which is recommended for vehicle collision sensing draws 1.3 mA. If it were the only load, it would take on the order of ten years to discharge a car battery, which is well below the self-discharge limitations. I don't know if lower power sensors are available, or if some rule like "enabled for 24 hours after the car is run" would be appropriate, but lack of power for the sensor does not seem like a good reason for the airbags to be disabled.

Bluetooth devices actively searching for devices to pair with could plausibly draw significantly more power than that.

[1] http://www.analog.com/static/imported-files/data_sheets/ADXL...