Note that these iTunes information fields show the data that was present for the original purchaser. They can be changed or even deleted. However the original info is still watermarked into the file, even since the DRM was removed by Apple. You can confirm this by looking at the file in iTunes after making any edits or deletions. It will still show the original purchaser ID and dates. This info cannot be changed with mp3tag.
Are you sure? Do you have any source for this?
Is this information not just saved in iTunes database, linked to this file?
I can't imagine that Apple really creates a unique watermark into the files (the audio-part or metadata?) for every single purchase.
Sorry for the late reply, Saturday = outdoor chores day
There are already some other replies that have links to some backup info. A simple way to confirm this is to copy one of your files downloaded from iTunes to a new temporary location. Rename the file to something you can easily identify to make sure you work with the copy. Delete all of the iTunes related tags. Change as many of the other recognizable tags like title, album, Artist, etc. as you want. Then scan the new file into iTunes. All of the tag changes will be reflected, yet the original owner and and purchase dates will still be shown in the file info section.
There may be other ways to make changes to this info, but it is definitely something Apple has left intact for a reason. I would have to believe that even without any DRM in place, there is still some agreement with the labels that this detail is kept intact.