I'm not sure you understand, it's exactly what the FBI were after. Unlocking a phone IF it is encrypted IS decrypting it. From an objective point of view the reason doesn't matter it's building the ability to action the request which in this case is one in the same.
Apple really have no obligation of vested interest in unlocking the phone. If it could provide the photo's another way then sure what ever, but the law surrounding ownership in the US around a deceased child I could guarantee you would not cover encrypted data. Technically the father has the data in his possession on the phone physically he just can't access it due to the function of the device, all apple would be required to do is provide the data if it was backed up on their services, there's no obligation to provide it un-encrypted as they did not receive it as such.
I feel for the man I really do but I don't believe what he is asking can be done and as cold as it is I don't think it should be done. If Apple were to compromise their encryption in that way it would be a disaster waiting to happen.