That's because they were never officially released, but the owner of the account didn't delete or remove them from the marketplace.
When a song passes at creators it creates that xbox page to purchase them online. Then it kind of goes into that limbo state to make sure there's no missed issues from testing. If nothing is found then Harmonix 'officially' releases them by letting them populate the in game stores every Thursday. If a song has an issue and needs to be pulled then you have to go in to the song page on creators and unpublish it, then delete it.
If you don't do that then the song will still sit out there even if you never release it.
Once they're unpublished they can not be downloaded again (IE Guitar Sounds, and few others) even if you have previously purchased them. This is also why you can find test songs and chunks of songs from when Harmonix was testing the pipes. They passed at creators and created the xbox web pages, but were never 'officially' released. Not that you would want to buy these test songs as they're just bits and pieces of other songs, but I digress.