As far as I know, these are from the original multitrack tapes. There is a lot of stuff out there in this category, including stuff by Queen, The Jackson Five, Beatles, etc.
That said, there is new software coming as an update to Melodyne that will allow you to manipulate the individual parts of a single wave form. In other words, you could put in a stereo mix of a tune and actually change the notes in the guitar part. It's pretty amazing. However, it's not fool-proof and having the multitracks would still be far easier and reliable.