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.