Tim Willis didn't just get paid to work for the FBI, he also had to adhere to their guidelines.
The guidelines include(amongst others):
1) Can't conduct any unlawful acts
2) Can't present themselves as employees of the FBI
3) Can't disclose information
However, Willis smoked pot with an elected official in 2004. The FBI confronted Willis in 2005 about the drug use and he admitted to act. The FBI did not do anything about the act.
According to Burns, the way they found out about the use of marijuana was not through video surveillance, but through a rumor. A woman approached one of the undercover agents and mentioned that Willis was distributing drugs. The FBI confronted Willis about this accusation. He denied it and said he only "smoked" marijuana at the elected official's home. They did not administer a drug test.