What about when Russian hackers shut down American hospitals and can't be extradited? Who's going to hack back and enforce the law? The FBI can't do it.
The original author said: "I don't want the NSA to perform domestic civilian law enforcement functions"
In other words, the FBI (and police, etc) perform domestic law enforcement, following criminal procedure. The NSA, CIA, etc handle non-domestic (foreign) threats, in whatever manner they see fit.