It's weirder than that, you aren't really paying for the items directly from Sony's platform when you play on the PS4, you're buying V-bucks, Fortnite's currency, and that's what you use to buy your items.

For one, the solution should be keeping two different V-Buck wallets: one for Sony, and another for everyone else, legal issue solved, moving on. So, because it's not happening, there must be something we're missing here...