Had a similar experience. We put credits onto my niece's account based on activities that were planned. Then the cruise went from FL/Bahamas to Canada and there were no planned excursions through
DCL (bye bye cabana!). So the extra amount that had been put on account prior to sailing (this is not a "new" thing, you and others have always been able to gift onboard credits to yourself or others) was refunded afterward (we had even prepaid gratuities so there was no charge to the onboard account for those). It turned out to be a few hundred dollars and we got the refund pretty promptly.
So no, this isn't a scam DCL is trying to pull to get extra revenue. This isn't even a DCL thing, you can gift onboard credits with most cruise lines.