The truth is that a lot of loyal customers are boycotting Starbucks right now over unionization and their stance on Palestine. So Starbucks is trying to bring them back physically into the stores and are using the balance on their cards as leverage. Starbucks has already uncharacteristically been running several promotions to get people back to their stores, with the red cup and the Stanley thing.
If Starbucks has to return the customers’ unused balance without bringing them into the stores, then they may have lost those customers for good.