This is called a float business in finance. Starbucks has more than a billion dollars in unredeemed balances, and they make ~$200 million per year in interest with this cash. They're basically a bank with a coffee shop side hustle.
A quick Google suggests in 2016 it was $1.17B, and earned $21M, or 1.79%.
If they're intentionally causing the customer to have an unspendable balance, knowing that it's making them $200m/yr, how is that not fraud (or some kind of crime)? I'd expect atleast CA would do something about it.
How are they getting 20% on a deposit that presumably could be called up at any time, and how can I get in on it when the stupid "High Yield" accounts I can find top out at around 4%?