Because they knew they would get relegated they tried to "defraud" (wrong choice of word but) the system by going before they had to so therefore avoiding any penalty.
The same thing happened to Chester and I think Rotherham as they did the same.
It won't happen to Portsmouth for two reasons 1) They had to go into administration when they did or they would have been wound up in fact they tried as longs as possible to avoide it. 2) When they got penalised they had a very real chance of staying up.without the penalty they would have finished on 29 points only 6 behind West Ham.
So it was in a very real sense a proper penalty, whereas Southampton would not have been penalised as it wouldn't have altered the outcome of thier relegation.