Unless the idea is to keep Americans uncertain regarding the origin of the attacks. If there were widespread sniper attacks it would be obvious to all that terrorists are the culprits; as it is the media is going to great lengths to avoid even bringing up the possibility.
The way I see it, terror cells could continue to switch modes (anthrax, sniper attacks, etc) but remain on a fairly small scale in order to continue killing Americans without getting caught. The calculation on their part may be that they can't expend operatives on suicide killings.