One aspect of that security plan is to ensure that all persons entering the site are screened for weapons or other items that may pose a security risk. Once inside, any disruption of the event that does not pose a direct security risk to the safety of a Secret Service protectee is the responsibility of the event sponsor and, in turn, local law enforcement, in accordance with local law.
The Secret Service has a longstanding policy of recognizing and respecting the constitutionally protected right of the public to free speech and expression. We make no distinctions in our actions based on the purpose, message, or intent of any individual or group unless that speech or expression communicates or poses a threat to the safety of our protectees.
-- United States Secret Service
Source: U.S. Secret Service