So many ponds (to drain), so little time...
The perps loved that little pond-draining exercize. Especially having led the FBI into it. What a fantastic plot. Think thru the FBI's options- No matter what the FBI did, they would come out looking like fools. It even provides never-ending grist for the make-fun-of-the-FBI contingent.
The FBI learned that it was outclassed in strategy and outmanoeuvred, and they are afraid that if they don't watch their step, it will happen again. The FBI do not wish to be made to look like fools in the anthrax case once more.