Now what is the Central bank of China going to do with $20M USD? Request it be delivered in $10 and $20 on a shipping pallet? Nope they will park it into ....
Fort Knox had to be emptied somehow.
For those with LOTS (i.e. tens of millions or billions) of dollars there really is nowhere to "park" it other than in a treasury bond. So even when the FED drives rates downs these institutional and governmental holders of dollars will still keep buying T-bonds and supply vs demand it will drive interest rates negative.
Because for a long-ish time gov't debt was a safe and reasonable place to store wealth it does follow that some people (not that many) will continue buying them even after they stopped making economic sense. However, this is akin to saying a famous restaurant doesn't have to serve good food, can get away with reheated stale pizza because it's famous anyway. This sort of bs doesn't work long term.
As a complete aside: putting a 5mn BTC buywall @ $200 each up in MtGox might actually be the most rational use of a billion dollars currently. You'll end up with 2-3 mn BTC and the price will be permanently in the hundreds. Profit.