If the argument here is essentially that we should trust the president to know better than the legislature and courts, why don't we just go with absolute dictatorship for all purposes?
The power the President is entrusted with here is solely to remit punishment. The idea is that this is an area where going overboard is less harmful than the alternative.
To add, it's easier to throw somebody back in jail if they aren't rehabilitated than it is to get someone out of 400 year sentence if they are technically guilty, no matter how extreme or ridiculous the prison sentence is for the crime they committed.
