What would be the reason for this Act?
edit - Here is a detailed desciption I found
Posse Comitatus Act
Source: G-OPL
"POSSE COMITATUS ACT" (18 USC 1385): A Reconstruction Era criminal law proscribing use of Army (later, Air Force) to "execute the laws" except where expressly authorized by Constitution or Congress. Limit on use of military for civilian law enforcement also applies to Navy by regulation. Dec '81 additional laws were enacted (codified 10 USC 371-78) clarifying permissible military assistance to civilian law enforcement agencies--including the Coast Guard--especially in combating drug smuggling into the United States. Posse Comitatus clarifications emphasize supportive and technical assistance (e.g., use of facilities, vessels, aircraft, intelligence, tech aid, surveillance, etc.) while generally prohibiting direct participation of DoD personnel in law enforcement (e.g., search, seizure, and arrests). For example, Coast Guard Law Enforcement Detachments (LEDETS) serve aboard Navy vessels and perform the actual boardings of interdicted suspect drug smuggling vessels and, if needed, arrest their crews). Positive results have been realized especially from Navy ship/aircraft involvement.
Source -
http://www.uscg.mil/hq/g-cp/comrel/f...Comitatus.html