They are banking on precisely this kind of “common sense” rationalizing of removing civil rights.

The civil right to be willing but currently unable to commit terrorism?

The civil right not to be led by the nose by a government agent to commit a crime. They are relying on people doing exactly what you're doing here - convincing themselves that if it was possible to lead them into it then they deserved it so the ends justify the means.

Yeah I get your point, and I generally agree for minor offenses like buying drugs or prostitution or whatever. But if someone can trivially be convinced to murder a bunch of people then the world is better off with them locked away.

I do see what you're saying. However this debate isn't unique to this specific scenario. The exact same reasoning can be applied to other violations of due process. Historically that was viewed as a particularly dangerous slippery slope and thus we have the concept of fruit of the poisonous tree, despite that this can easily result in (for example) a mob boss walking free. Perhaps caution is warranted when considering abandoning well established principles?

https://en.m.wikipedia.org/wiki/Fruit_of_the_poisonous_tree