When they receive a lead, they should follow up on it. They received the lead from one of Hatfill's aquaintances who stated Hatfill had told them how he would get rid of such evidence. Just so happens they found the glove box in the pond within miles of his house, disposed of in the same manner he had previously described to the aquaintance. What do you expect them to do? Ignore the lead just because it is connected to Hatfill in some manner? I just don't understand your logic.
While I am not saying he is the guilty party, it is a lead worth looking into to find the culprit. The anthrax used in the mailings was traced to weapons labs at Ft. Detrick. Also, if I remember correctly, the anthrax that was sent to Daschle's office did not disperse as it normally would have because it was damp, another indication of being assembled under or around water. Add in the glove box found in the pond and the vials and I would say they have all the justification in the world to investigate this pond further.