Back in the days of
Hot Type I could see disposing (melting

) of the copy.
When the started using photo Offset printing, I would think that the Film would be warehoused (1 thick sheet per page is still a LOT of shelf space), at least for the duration of the contract to allow quick turnaround on reprints.
With Direct to plate printing became common, disk space had already fallen, so I see no reason the source files were not stored for the contract period.
But they didn't. Just like many BPH waited to the end of the contract to change the status from "Out of Stock" to "Out of Print"
(IIRC OOP status, rights reverts back to the Author )