And in this case, that difference allowed the sniper who ultimately shot Crooks to be unaware of the threat long enough for the loser to get off six or seven shots, to wound Trump, kill a bystander, and critically injure two other people. The handing over of part of the critical area to local police who can not instantly communicate with Secret Service command, is a critical flaw in efficient security.
To be fair though - it must be an absolute nightmare trying to protect a figure like Trump in a situation in which thousands of members of the public - all potentially armed have access to an open air site like that and allowed to get near the potential target. The potential for acts such as Crook's, or equally dreadful, the potential for an agent to misinterpret some innocent, weird situation and shoot a harmless fan is high.
EDIT:
I think it might happen that open air events like this are not so common in the future. You can control who gets into a building and search all entrants. The potential target is only exposed as he gets out of an armoured limousine and steps rapidly into the building, but an outdoor event over a large area is far more dangerous.
Kimberly Cheetle will be out of her job within days.