I find this to be a very strange rule indeed, why not have it for either all or none, instead of just some.
It is for Grand Slams as well.
I find the 0-point penalty rule rather volatile too. That is because if a player withdraws without mentioning a good reason from a ATP 500 event, he may be penalized and I have no way of predicting that.
So, in case of ATP 500 events, it's not just for missing the commitment.