surely tons of people do things like climb nearby buildings and maybe even setup like tripods to photograph or video politicians. The secret service isn't just gonna brain some guy 150m out if there's any chance he's not doing the damn thing.

Evidently they don't because I'm sure people do what you describe but how do you, as a country, get to "Cop starts shooting at handcuffed perp in his own car because an acorn dropped on it" vs. "Our ROE says we can't shoot first to protect whoever the secret service protects"?