Why Airbus would only supply this information on the ground is a little puzzling.
Err, it doesn't! With AUTO selected, the transponder operates in ALL modes when the aircraft is airborne, but on the ground it only operates in Mode S. That's because the Advanced Surface Movement Guidance and Control Systems (A-SMGCS) used by ATC at some airports uses the Mode S 'squitter' to obtain aircraft identification and position. The other modes are disabled on the ground so the transponder doesn't respond to transmissions from the Mode A/C Terminal Area Radars.