In the same way we take it for granted that a SWAT team could kill any of us pretty easily. They have (and probably should have) the ability, but the authority to use it should be very limited and subject to stringent oversight.
Sanctions are kind of like the tax agency situation - if you're going to wield them as a national security tool, then you want to be able to enforce them, otherwise they're relatively meaningless. And enforcing them necessarily involves watching to see if your country's citizens are evading them.
The type of oversight applied to military and foreign intelligence action is fundamentally political. Whether and how to impose sanctions against, spy on, or invade another country are questions for congress and the president in the US and rarely involve the courts.
The type of oversight applied to law enforcement is fundamentally legal. Courts apply existing law to the set of facts in a given case. Congress and the president do not get involved directly and any attempt by political officials to influence the outcome is scandalous, if not criminal.
So should the NSA hack Iran's immigration database to see who's been going there? Perhaps. Should anything discovered that way be admissible in a criminal trial? Almost certainly not. Should the NSA tip off the FBI about domestic crimes? Mostly no, I think, but perhaps in a very limited fashion when the crimes involve national security or foreign policy. Gathering evidence that can be used in a criminal trial would still be the responsibility of the FBI.
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.