Continuity mistake: While Kelly is being interrogated in the interview room by Nick, her hair changes between shots. Close-up it hangs loose. In the wide shots both sides are tucked behind her ears.
Room Service - S6-E2
Continuity mistake: When Warrick is in the hotel room at the start, the corpse's eyes alternate between open and closed.
Continuity mistake: When Nick "attacks" the kidnapper in the interrogation room, a single tear falls down from the boy's left eye. When the shot changes to a different angle, the tear is coming down from his right eye.
Answer: He doesn't have the authority to arrest him - when anyone is arrested in the show, a police officer does it. Also he tells Ty that he is going to be submitting his case findings to Ty's insurance company, who would no doubt contact the IRS, who then would have him arrested for fraud etc.