Here's a sniper guy analyzing the situation - nothing new really, except one bit: he says that it's actually the snipers' job to scan for possible sniper positions of an assassin before the event. They would basically go on the stage and look around to identify possible positions. Which would make sense given that they are the experts on these things.
If that's true, one has to ask about the involvement of the counter sniper team - were they Secret Service? This doesn't mean that the 2 guys in the video were in on it necessarily, but at least someone must have seen to it that they get the right orders as to where to focus their attention, and have someone who's in on it do the initial assessment or break the chain of information somewhere.
I think it comes down to area of responsibility. The police sniper team were probably looking further afield than those buildings, because those buildings were in the area of responsibility of the local police, a responsibility they obviously failed to live up to. Seems to me that in mass shootings over the years, the local police were often infiltrated in some way and were ultimately blamed.