While the Fed buys Treasuries depressing the yield, they don't buy all of them. What they don't buy at auction and the trades on the secondary market are subject to the attitudes of investors who are becoming increasingly worried about losing principal as Dollar shrinks. IMO the Fed will do more QE soon, in the hope of improving the economic metrics before Fall elections. However, one can argue that a reason that TBT was $50+ on somewhat higher rates in Winter and into Spring was because of bond investor reaction to the earlier round of QE. We'll see.