A search warrant isn't a license for law enforcement to break the law; it's a license to lawfully conduct a search and seizure based on probable cause of the commission of a crime. Warrants exist because of the Fourth Amendment, which /specifically/ deals with searches and seizures; warrants have nothing to do with a nonexistent general rule preventing law enforcement from breaking any law. Law enforcement can and does break the law all the time to conduct sting operations regarding drugs, prostitution, child porn, hitmen, etc.

For instance, the DEA can buy drugs in order to trace and later apprehend drug suppliers. They don't need a warrant to do this (though information gained from such a sting operation may later lead to a judge issuing a search warrant).