Not sure at what stage it became a feature, or if it always was on these, but there is a manual way of disabling the immobiliser. In the handbook pack there should be a security code (should be in a sealed envelope if it hasn't been used or removed) and there is an instruction in the handbook that tells you how to do it. It involves a whole bunch of pressing buttons, counting flash's on the dash and turning the ignition on and off.
I thought I had a problem with the immobiliser on Mrs D's 316i a few months ago and went throught his, even though I had the key. I understand that they can be a bit bothersome. I ended up having this one removed and installed a non BMW immob. It didn't solve the problem, but that's another story..!