“The scandal is that the Secret Service allowed any vantage point, especially so close, to be left unsecured.”
It was secured. The SS counter-snipers had their sniper rifles on tripods aimed at the roofline where they killed the perp. We know this because videos show the SS counter-snipers looking in the direction of the perp and when the perp starts shooting, they shoot back without re-aiming their sniper rifles.
The simple fact he got up there with a rifle and fired shots is prima facie evidence it was not "secured."