Tony: All NPAs have to be flown manually after MDA (by definition) On the Airbus the A/P will disengage itself at MDA-50ft anyway.
On an ILS the flightpath control is via the elevator tracking the GS with the speed being maintained by the ATHR. After MDA if the F/Ds were continued to be used they would be following a command which may or may not be appropriate to fly the approach. Hence they are switched off.
If you are still having trouble with this concept consider this scenario:
Visual after MDA, F/Ds still on , A/P off and OPEN DESCENT pulled on the FCU.
If you continue to fly the PAPIS - what speed will the aircraft get down to?
I am deliberately not telling you the answer to get you to think it through yourself!!