One of the differences in the US is that evidence obtained by espionage is rarely admissible in court. Often spying bypasses constitutional and legal protections and renders evidence unusable. In the US, cases get dismissed all the time because evidence was "fruit of the poisonous tree" where law enforcement attained evidence unconstitutionally or illegally. This evidence is not allowed in court. This often renders the work of agencies like the CIA, DIA and NSA fairly useless BUT NOT ALWAYS useless for law enforcement. A lot of the problem with FISA and PATRIOT act is they create loopholes in the line between espionage and law enforcement.
