Looks like Microsoft is going to allow pre-loading on the Xbox One after all! Looking on the Xbox One store you can see the game is now available for digital preorder as well as new text that says:
"Xbox One Game expected to release on 9/9/2014. You may download the game from Xbox Live before then, but it will not be playable until 12:01 AM PDT/3:01 AM EDT on the release date."
Update: Microsoft clarified the preloading and preordering for Destiny: "We're thrilled to make Destiny available for pre-order and pre-download for Xbox One owners around the world. This option was available for a short period of time before the official pre-order window opened. We'll have more details to share soon about the pre-order and pre-download window for Destiny."