They can't unload them. To do so would man the U.S. would have to pay higher interest rates to finance its debt, which would crash its economy, which would cripple its ability to buy Japanese goods.
They are stuck with financing our overspending and underproducing. We've got them just where we want them.
I would guess, though, that they are hoping to help engineer a gradual decline in the $US, meaning a gradual decline in our ability to buy from them, which would give them time to find other markets.
Hard work often pays off over time,
but laziness always pays off right now.