RealClearPolitics is reporting the following:

"But a source within the Secret Service community tells RealClearPolitics that the agency rules of engagement in this situation are to wait until the president is fired upon to return fire.

'You want to take a shot then find out the guy was holding a telescope?' the source suggested. 'The Secret Service is by nature reactive…and you better be right when you do react or you’re f-----d.'
 
The Secret Service protocol requires that a counter sniper aware of a potential shooter to radio directly to intelligence division team to respond and investigate. In this case, the investigation may have been cut short by the shooter firing his weapon, so the counter sniper then fired as quickly as possible in return."