You have to look at what the agents are authorized to do under the 2 laws specified in 118.
Any person who knowingly and willfully obstructs, resists, or interferes with a Federal law enforcement agent engaged, within the United States or the special maritime territorial jurisdiction of the United States, in the performance of the protective functions authorized under section 37 of the State Department Basic Authorities Act of 1956 22 U.S.C. 2709 or section 103 of the Diplomatic Security Act 22 U.S.C. 4802 shall be fined under this title, imprisoned not more than 1 year, or both.
22 USC 2709
a) General authority Under such regulations as the Secretary of State may prescribe, special agents of the Department of State and the Foreign Service may—
22 UC 4802
(a) Security functions
(1) The Secretary of State shall develop and implement (in consultation with the heads of other Federal agencies having personnel or missions abroad where appropriate and within the scope of the resources made available) policies and programs, including funding levels and standards, to provide for the security of United States Government operations of a diplomatic nature and foreign government operations of a diplomatic nature in the United States. Such policies and programs shall include—