Amazon has put the $399 version of Xbox One, the one that comes without Kinect, under review. This probably means that their customers have indicated an issue with the product and they are verifying their claims.
Usually it’s not really a big deal but you can purchase it from third-party sellers. People have been reporting coil whine/noise issue so it may have to do something with that.
Here’s what their message states:
While this item is available from other marketplace sellers on this page, it is not currently offered by Amazon.com because customers have told us there may be something wrong with our inventory of the item, the way we are shipping it, or the way it’s described here.
We’re working to fix the problem as quickly as possible.
We will keep you updated on this.