Actually they don't, that's not how the bond market works.
They spent say, $2T for a stack of bonds with a total value at maturity of say, $2.2T. The 200B profit is what they might eventually get instead of interest. Or not. If the plan is to raise interest rates when sell-time comes around, they may want to sell the bonds at a loss as a way to kick rates higher.
fwiw, right now the fed's holdings is ten percent total federal debt. That's the same percent when Reagan was president. So much for QE money printing.
Why the low QE? We’re credit starved. Someone said they’re trying to keep a 1.3/1 parity with the Euro.