I will use Glenn's argument against this point.

There were no children. The officer sold nothing.

Sarcasm aside, the reason the officer is "above the law" is because he is acting as an agent of the law. The man provided reasonable suspicion that he would participate in a criminal act; this grants law enforcement more power over this one specific person in an attempt to avoid a situation where he finds a real child broker.

It's pretty much social contract; we sacrifice rights in return for services from the government. Personally, I have no issues with granting the police the right to have heightened power over those who have proven intent.