The developer page allows downloading the exact same package file that you previously uploaded. There is currently no mechanism, within Roku or anywhere else, to decrypt a package other than when it is executed on a Roku device. Adding the ability to download the decrypted package from the developer page would require implementing a new system to decrypt files without a device, which would compromise application security with very little benefit. The only advantage would be a last ditch backup for a developer who somehow lost his source. That's never been the intention of this feature.
--Mark