Because just as the Federal Reserve can arbitrarily expand its balance sheet to purchase assets, it can also arbitrarily shrink its balance sheet when it sells

Because just as the Federal Reserve can arbitrarily expand its balance sheet to purchase assets, it can also arbitrarily shrink its balance sheet when it sells those assets. In other words, the Fed can print money to buy assets, increasing money supply, but given the large increases during the recovery, if inflation picks up the Fed might want to shrink the money supply. It can only do that by selling assets and destroying the money it receives.