Dr. Weir and Col. Everett are standing on the balcony watching the aerial battle, along with two marines armed only with surface-to-air rocket launchers. Were either these marines to fire their weapons to destroy the approaching dart, they would incinerate everyone on the balcony. The situation is made worse when Col. Everett, in an attempt to protect Dr. Weir, pushes her into the area directly behind the launchers.
Manually aimed AA guns are obsolete. They needed to be phased out shortly after the end of the Second World War, with the advent of jet planes, which made them ineffective.
Sheppard states that the life sign detector cannot distinguish between wraith and human, but earlier in the season when on the wraith hive ship the wraith show up as red squares and humans as blue circles.