Different contexts require different paradigms. For example, you can't operate in a war if you require your soldiers to arrest everyone who is shooting at them and collect evidence for an eventual trial. You also can't operate as even a regional power (let alone a global superpower) without keeping tabs on your adversaries.

The difficulty is in structuring agencies that have to follow different paradigms and determining how they should interact. One key bifurcation is supposed to be that intelligence services face outward, dealing with people who don't have constitutional protection because they are not residents or citizens. That outward focus has gotten muddied by the focus on preventing domestic terror attacks, the reality that much of the internet's physical infrastructure exists within our borders, and the difficulty in automatically determining the nationality of online personas.