Because the passes DFTW gives you are like promo tickets. They have a $0 value in Disney's system. They work the same as a full-price basic Annual Pass, but they are not recognized by the system as having a full value that can be applied toward any other pass.
It would be like if you won free balcony-seat concert tickets from a radio station and then tried to apply them toward buying orchestra seats. They are freebies, not actual full-price tickets.
I know it's a drag, but, hey, they're free!