What justifies your expectation that the store is required to be responsible for these cards?

Let's look at it from the store's perspective. I'm going to ask that you assume positive intent here:

• A player completed the pre-release and left a box of cards on the table. This is not out of the ordinary. Players leave behind the extras they drafted/opened in a sealed pool all the time. I have done this a large number of times, cherry picking the rares/mythics/foils/uncommons that I wish to keep and leaving the rest on the table.
• This happens at midnight. After the pre-release is over, and the store is closing. The player has collected their prize, returned the basic lands, and has left the store.
• Another player notices the box, and asks for possession of it. From my experience in playing in a lot of limited events, this is not out of the ordinary. Some players just don't want to end up with a large pile of extra commons/uncommons that they have no further use for.
• The store (and the player who asked for possession of these leftovers) may assume that whoever left these cards has already picked through everything they wished to keep and just abandoned the rest (this is, again, very common. I've done this multiple times).
• You cannot expect either the store or the player who asked for possession to have actually looked through the cards to determine whether or not they were abandoned or not, especially when it's past midnight.

The worst you should be able to call this is a lapse in judgment due to prior experience, and I'd say you'd be grasping at straws even then.

If this were a constructed deck, then I'd be inclined to sympathize with you.