I believe the only assets the Japanese could sell to prop up the yen would be US govt debt. If they did that, the US bond yields would go higher and the cost to the US to service its own debt would go up. Not good for the US.

So by selling Euros and buying yen, the US doesn't negatively affect the US bond market. The US is just swapping out one foreign currency for another without negatively impacting US bond yields. This is a better deal for the US. At least I think that's the strategy.