It deliberately kept the question vague, even though the United States knew that this vagueness and unconditional nature meant that the Japanese felt they would not be able to accept it.

Here is how the War Department Office of Planning put it before the Declaration was finalized:

The primary intention in issuing the proclamation is to induce Japan’s surrender and thus avoid the heavy casualties implied in a fight to the finish. It is almost universally accepted that the basic point on which acceptance of surrender terms will hinge lies in the question of the disposition of the Emperor and his dynasty. Therefore, from the military point of view it seems necessary to state unequivocally what we intend to do with regard to the Emperor.

The Secretary of War suggested it read:

The occupying forces of the Allies shall be withdrawn from Japan as soon as our objectives are accomplished and there has been established beyond doubt a peacefully inclined, responsible government of a character representative of the Japanese people. This may include a constitutional monarchy under the present dynasty if it be shown to the complete satisfaction of the world that such a government will never again aspire to aggression.

Whereas the Joint Strategic Survey Committee changed the last line to read:

Subject to suitable guarantee against further acts of aggression, the Japanese people will be free to choose their own form of government.

And the Office of Planning suggested:

The Japanese people will be free to choose whether they shall retain their Emperor as a constitutional monarchy.

All of these sorts of things were rejected. The only line about Japanese governance in the final declaration was:

The Japanese government shall remove all obstacles to the revival and strengthening of democratic tendencies among the Japanese people.

Which does not sound very compatible with an emperor system on first glance; at the least, it gives no reassurance to those who feared that the US planned to dismantle the imperial system (which in their eyes meant the dismantling of what made Japan "Japanese"). Again, the planners knew, from the MAGIC intercepts, that this was the primary sticking point with the Japanese. The decision to remove anything that would give "reassurance" was made by Truman and Byrnes, in the opposition to many others.

Draft language comes from Hasegawa, Racing the Enemy, 146-147,