Alright, I admit that from a tactical point of view, if the local cop got shot it would have alerted other security and maybe no one would have died … though at such short range, it would be likely that the cop died instead of the firefighter in the stands.
That said, I still don’t blame the cop – who I presume never had Secret Service training – for taking cover when someone aims a gun at them. That seems like an instinctive reaction that you’d need serious training to overcome. I expect it to take a few seconds at least to figure out how to approach the situation before you stick your head back up for easy targeting.
Better solutions might have been to either: a) have at least one person on that group of rooftops and every/any other roof-group in advance, or short of that b) when rally-goers said there was someone on the roof with a gun, radio that info FIRST and THEN look. It is possible that the latter happened, but no one passed the information forward and the cops haven’t owned up to the failure yet, but IF that happened, that sort of detail would tend to come out later rather than immediately.