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%?

Do they include expiring credit in that figure?

Most of the time they have a buried clause that says that you forfeit all of your credit or get charged an inactivity fee if there have been no account transactions or no credit added for 12 or 18 months. Same reason why you should never buy gift cards.

large businesses have large cash borrowing needs. if they borrow for free from their customers, it reduces the other borrowing they would need to do, so the rate to use is not what interest rate is available to you, but rather how much interest that Starbucks would need to pay for loans that size. Furthermore, whereas dividends are taxed twice (once as profit for the company and again as regular income to the shareholder), interest is a tax deduction to the company (which decreases their taxable profits) and for a percentage of debtholders that interest income is also taxed advantageously.

probably doesn't come up to 20% (unless Starbucks is in junk bond territory) but it's higher than the investment rate of 4% that you're quoting.

They may buy bonds or something like that.