I think Pens Fan's guess that he was buying merchandise to resell seems very plausible, though you'd think it would've been in the news story if he'd put up umpteen million eBay listings right after the trip. I wouldn't think that'd be hard to figure out if they searched his home.
Don't know about using Disney gift cards at other shops in Disney Springs -- my understanding is that they're independent retailers, so I wouldn't expect to be able to spend them outside the Disney-run stores there (World of Disney, whatever the Christmas shop is called...hey, Art of Disney!). I bet you could spend a lot of money very quickly at Art of Disney. But yeah, it does beg the question of how no one else in the party noticed that he was buying that much stuff. Maybe he told them he got some kind of deep discount/prize for the lodging and then snuck off to make merch purchases? I don't even know, man.