You pay a tax once for a given amount of income, you pay interest every year. This whole thing only makes sense if they can eventually repatriate their earnings at a lower tax rate than they have and if the difference between that potential future lower tax rate and the current tax rate is larger than the accumulated interest paid until this moment.Interest rates on corporate bonds are historically extremely low right now.
So issuing debt now allows them to finance things like the capital return program and dividends more cheaply than using their foreign cash pile (the bulk of their savings) and bearing US tax losses when they repatriate those earnings .
tldr: Interest rate % for bonds are lower than tax rate % on cash