I think the point is democratic states should (need) not have intelligence agencies with unlimited powers. They corrupt the state as they establish themselves as a central pillar of power (next to the military and the government).
They don't even have any historical roots in western political systems. The country with the claim to that is Russia (http://en.wikipedia.org/wiki/Okhrana), where they were commonly used to quell internal unrest and combat opposition.
The role of an internal secret police goes back at least as far as the Roman Frumentarii[0] and I think you can trace a direct lineage from Walsingham's[1] operations to the CIA/NSA.
They don't even have any historical roots in western political systems. The country with the claim to that is Russia (http://en.wikipedia.org/wiki/Okhrana), where they were commonly used to quell internal unrest and combat opposition.