if Ferrero had done the same as Isner, he would have had a penalty-zero as well : this penalty-zero is the same, whether committed or not, some non-committed players got it in the past. The penalty-zero for commitment players is only when it's certain after one ATP500 tournament that they won't play 4 ATP500 tournaments in the calendar year
Isner had no good reason at all to withdraw from Monte-Carlo : no injury that took him out for one month (he was playing Houston that week), didn't come on-site (he said he wanted to practise longer in the USA) to have an injury checked (Gasquet the Frenchman was practising in Monte-Carlo when he withdrew) or to make some promotion "hey I'm there !") : the way Isner announced his withdrawal I had guessed he would get a zeropointer. He just said that he wanted to rest
I don't remember Ferrero's case exactly but I guess he had an injury
But well to say the truth, apart from obvious cases like Isner's, usually players manage to avoid these kinds of zeropointers without any "obvious excuse" for us. For instance, we never kn ew how Djokovic avoided it for one tournament (Rotterdam ?) last year.
You can check the previous rankings thread : we had talked on that topic and precised the rules in very long extent, but practically for your work just don't give these kinds of zeropointers in advance.
Just do it when it's obvious that it was purely a "comfort withdrawal" AND that the player didn't come on-site when he announced it (Isner was in Houston and stayed).
Besides, some players sometimes get these kinds of zeropointers then manage to have them cancelled a few weeks later somehow, appealing to a kind of "ATP court" which seems to be very nice to players to say the truth. Also they can have them cancelled by somehow engaging to play one more ATP500 tournament than expected.