Since we are free to talk about guns and police - this should be ok.I am not a police officer, and I cannot fully comprehend what they are subjected to. However it looks very bad to see video of LEOs hiding behind trees and cars while shots are being fired. I can understand that there might of been under orders to secure a position, but nonetheless - it looks bad.
As an 18 year cop, I'll address some of your issues. Prior to Columbine High School, the "accepted" police practice was to lock down the perimeter and wait for SWAT. Columbine presented most agencies with the fact that they had to address what is now referred to as an "Active Shooter." An active shooter is when someone has used deadly force and has unrestricted access to additional victims. Once the person is locked down in a small area, it becomes a hostage situation (unless he continues to fire, in which case it remains an active shooter).
The training now is, the first 4 to 6 officers arrive and create a "contact" team that then goes in and hunts the shooter. In a place like Blacksburg where this happened, you aren't going to have hundreds of cops available, only a few dozen, and even then the response time will probably lag. It isn't like the officers on the first crime scene weren't doing anything, they weren't necessarily free to leave. So, the first officers go hunt while the rest maintain a perimeter.
The images you see on TV are not the hunting officers, but ones who have a very specific and necessary job: maintaining the perimeter. A perimeter officer doesn't stand out as a free open target, but rather from a place of cover or concealment. In larger metropolitan locations, you may have four or five contact teams.
But as I mentioned in an above post, by the time the police were there, it was over. Long over.