I suspect the A5's are slightly different as they use a key fob that is inserted into a slot to the left of the steering wheel. Therefore every A5 fob is the same, they are just coded differently. The only difference with the advance key with the A5 is that you don't have to insert the fob into the slot to start it.
Scrotes just need to break into the car, disable the alarm through the OBD port, programme a black key fob using a device connected to the OBD port, and then drive off with a newly programmed key.
Since the S3 (without advanced key) uses the old fashioned twist and go key, I imagine it isn't possible to do it this way.