to me it sounds logical for the counter-sniper not to shoot first, it would have brought several legal problems on the table if the secret service killed someone first and asked questions second, there would definitely be lawsuits on the table against the secret service and the one who pulled the trigger.
I think the SS would have been protected from liability by killing a man with a rifle crawling on a roof towards a political leader. (At least here in Canada
intent plays a big deal.) On the other hand, now they do have a lawsuit (probably) for allowing that innocent civilian to be killed.