It is illegal to sell alcohol to someone under 21. So as part of an investigation they’ll send someone under 21 to buy beer at a store. But wait. Buying alcohol is also illegal if you’re under 21 (if not in your jurisdiction please don’t fight the hypothetical). How can cops break the law or induce someone else to in order to create another crime?
There are exceptions that allow law enforcement activity. The police are not breaking the law when an underage enforcement agent attempts to buy alcohol.
For example, in PA, Title 18 § 6308, section (e) is titled “Exception for compliance checks” and details the requirements for an under-21 “officer, employee or intern of the Bureau of Liquor Control Enforcement of the Pennsylvania State Police” to legally conduct enforcement operations.
In South Carolina, the law is Title 61 (Alcoholic Beverages), Chapter 4 (Beer, Ale, Porter, and Wine), Article 1 (General Provisions):
A similar section applies to the law regarding hard liquor.
Why use alcohol as an example? Police either buy illegal drugs or have others buy drugs and observe them all the time. Or pay for prostitutes. Or accept money as a fake hitman. It’s not legal to buy illegal drugs, or pay a prostitute, or enter a conspiracy to commit murder. But it is legal to do those things in the furtherence of an investigation.
There are many things law enforcement people are permitted to do in the course of their duties that a normal citizen is not allowed to do.
For example, when pursuing a traffic violator, a cop car can speed, run stop signs, go thru red lights, etc. And stop on the road, with flashing lights. And block traffic, and PIT another vehicle, etc. And speed to the scene of an incident. And use force to restrain someone, and pull them out of their car without their consent, and haul them off to the jail, without facing kidnapping charges.
We as a society have decided that the people we appoint to enforce our laws can break some laws in order to arrest criminals. But we have limits on that: ‘entrapment’, ‘excessive’ force, evidentuary exclusion, and so forth.
Since those exceptions are written into the law there is no breaking of the law to begin with. There are exceptions and affirmative defenses that are written into various laws that cover people and circumstances other than law enforcement.
Should be noted that there is a difference between what an LEO can do in the line of official duties, and what someone used by law enforcement can do. For example, in the statute I cited above, the people being exempted are not LEOs, but those people hired by law enforcement agencies to do the test buys.