One government motive would concern the liability issues arising out of an act of war. If the anthrax attacks can be passed off as the act of a deranged individual, that's one thing. If, on the other hand, they are an act of war, that's yet another.
Some well meaning but otherwise infinitely stupid individual over at FBI may have nothing more than that in mind.
The Justice Department and it's subordinate agencies have more than their fair share of such people.
Somehow I think there is more than one reason for this position. Politics may well have a lot to do with it.
Stay well - Stay safe - Stay armed - Yorktown