I suspect that the idea originally was to give some safety valve but if it is used more than a few times by a President, it makes a mockery of it and it should be removed as a power. How can a President ever decide that the entire legal process is flawed and their opinion is right? If the sentence was too long then change the sentencing guidelines.
It's not one that should be needed or acceptable, and had his successor been someone who seemed to respect law and order I'd have agreed with you, but in the present circumstances it'd seem crazy not to.