Somebody wanted to wake up the U.S. to the possibility of bioterrorism. Remember, before this all started, we didn't know just how effective anthrax was. The package that was sent through the mail was sufficiently sealed based on what we knew about anthrax to keep it from leaking through the package - it was only after the package was discovered that we found out just how effective the stuff was. In addition, the letter detailed that anthrax was contained in the package, and that the people exposed should probably take antibiotics immediately.