Door on catch incidents are a pain in the backside, particularly when the train gets stopped in the middle of a busy junction in the pitch black for the poor driver to take a block and start clambering round the unit finding the door and climbing up the side to get it to properly close.
Late running passengers see a door handle and grab at it to try and open the door as the train departs, causing delay and risking injury.
Passengers who've left it too late to get off and the CDL is locked, look ahead, realise its just big enough to get through, and try and alight via the open droplight.
The list goes on - in theory slam doors are fine, but the potential for misuse or human error just means from the perspective of operating a railway they won't be missed.