if they have yen-denominated debt, then they have added currency risk
if it is 1% of their cash position, they can just use some cash and buy yens and have those yens available.
if they have hedged the bond currency risk, then they might as well have hedged it after buying yens in the open market
if it's 1% interest, it is still probably more than the interest they earn on their cash position.
just saying...
(MAYBE, maybe, its a tax issue)
And, my bet is Buffett knows of some companies he could deploy that Yen in right now to cover the interest anyway. He's just lying in wait for better oppurtunities.
Now, there could be some other thing going on-- but taxes-wise in general U.S. companies issue US-denominated bonds to repatriate funds so I'm sure where you're going with that. My bet would be if it's not just for the opportunity, it may for their re-insurance reserves.