MP3tag doesn't seem to be able to handle imports of "various artists" CDs, either from FreeDB or Amazon. FreeDB handles compilation CDs by making the title "Artist / Title" and the artist "Various"; Amazon does it by making the title "Title - Artist" and the artist "Various Artists". In both cases MP3tag just imports this literally rather than moving the artist to the proper tag. I've tested this with the latest development version (2.37i as I write this).
Please see Utils > Compilation... at the freedb dialog and use sth. like %artist% / %title% as format string.
Ah, thanks. Although I don't see how to get that with the Amazon import.
You can use an action to split artist and title
Action type: Guess values
Source format: %title%
Guessing pattern: %artist% / %title%