The Fox Die-Hard Trilogy set lacks UPC codes on the discs. It's to keep them from being resold separately. A retailer could buy the box sets and separate the discs. THe price he pays is lower on the boxes.
Same thing with used shops. They usually separate out box sets.
This is usually done to prevent the set from being opened, then having any of the individual discs offered for resale or returned. My box sets of SCREAM, DIE HARD, PLANET OF THE APES and FRENCH CONNECTION all had "voided" UPC barcodes or no barcodes at all.
I think the main reason for this is that you can send the UPC away with 6.95 to get a free "extras" DVD as well as keep-box if you just purchase the JP3 DVD. I think they are preventing you from getting this deal if you have already purchased the boxset.