 The counter-snipers couldn't have had a clear view of the shooter. In Grimmer's article, he presents a bird's-eye view of the shooter in counter-sniper locations and shows that there is clearly a large tree blocking both the northern and southern sniper teams' view of Crook's position on the AGR roof. They couldn't see Crook crawling up the roof, even if the police below them are sounding the alarm. He's behind the tree.