The Fed invites you to front-run their trades

I love how the Fed announces the day before they come in to buy up Freddie and Fannie paper. The NY Fed just announced they will be in buying agency coupons tomorrow. I understand that they want to inspire confidence and get others to join in their trades to force down long-term interest rates (and in turn, mortgage rates), but it also gives the dealers a chance to push the market against them and make them pay up for bonds, leaving the taxpayer with a worse fill than they otherwise would have gotten.