Modern vehicles report OBD-II signals over Controller Area Network, which is used to link most the controllers in your vehicle. All cars after 2008 in the states use CAN for OBD-II.

Most manufacturers have the OBD-II CAN bus connected to the engine controller for diagnostic purposes. If you can gain control of the CAN bus, you can do some real damage to the car. Furthermore, the hardware must write to the bus to get OBD-II data (it's a request/response system), so it's not a read only device.