I don't, in this case. We can take a few things as given, yes?
1) There is no legitimate reason for an individual to attempt to blow up a piece of domestic infrastructure.
2) Any person who, given the materials, has no qualms committing the act in 1, is a criminal and should be taken off the streets.
This isn't even like most other stings. Drugs, speeding, whatever. There are legitimate reasons, law notwithstanding, to do both of those things. However, a person who takes steps to blow up a building and kill a bunch of people.. nope. There's zero question in my mind about someone who would do that. There is no legitimate reason. I suspect that the FBI will have a 100% success rate with these actions, and zero false positives.
What are the chances someone will be innocently caught in this dragnet? Pretty much zero. So where is the harm to society?