I suspect that there are two reasons that the AA suggest what they do.
One as George states is that it is idiot proof. And cross connection is probably the case of subsequent electrical faults
Two. once the positive lead is connected, connection of the negative lead onto the chassis would avoid sparks actually at the battery where you are proabably leaning over, without protective googles or have blown away any potential gasses.
Also, Jaguar should know! I agree, they really should