Continuity mistake: While Sara is looking through missing person's files, Grissom comes in and starts talking to her. One shot her hair is in front of her face, the next it's behind her ear. This continues throughout the scene. (00:22:05)
Continuity mistake: When Catherine and Warrick are testing to find the distance the shots were fired from, when Catherine shoots at the shirt that's 2 feet away there is no gunpowder around the hole. When Warrick puts the victim's shirt next to it, for comparison, there is now an inch of gunpowder around the hole.
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.