Unconditional surrender was what the allies had agreed to early in the beginning of WW2. To prevent the allies from making a separate peace the allies agreed that no nation would make a separate peace and only unconditional surrender of the axis powers was acceptable to end hostilities. Since Japan had attacked the USA prior to issuing a Declaration of War the USA was justified in insisting upon unconditional surrender. Once in the alliance with Great Britain and the USSR the US could do not less the insist upon unconditional surrender because that is the terms that the allies agreed upon in 1942.
As Japan I would not have gone to war in the first place.
Sun Tzu - A victorious General wins his battles before he goes to war, a defeated General goes to war then tries to win.