The FED are trying to keep the inflation at a steady level, right? If the interest are too low, the inflation goes up, and you risk too high inflation. This will lead to bad investments, and bubble economy. The FED are not trying to bring down the US economy, they are trying to make it as good as possible. Of course the ecnomy in short term will seem to go better, but in the long term history has shown that too high inflation is followed with bad investments, and therefore bad economics