At that point, Kraft is a "person of interest" regarding the investigation of a crime. Cops can legally ID a person of interest without identifying him as a criminal or even notifying him that he is indeed a person of interest. They perhaps had been told that a guy who looked a lot like Kraft has entered the place, left the place, and what happened in-between was on tape.
What if Kraft later says "That's not me on tape! I'd never do that. I was nowhere near that place. There are lots of slightly stocky older white men in Florida who look like the person on that tape".
Well, they are ready for that precisely due to the stop. The cops then have a record of him being in the vicinity of the crime within minutes of the time stamp on the tapes. They have a record of him giving identification, a record of the licence number for the vehicle, an ID for the driver. It is game, set, and match for putting RK at the scene of the crime.
I am not a lawyer or a police officer, but to this citizen, the actions of the cops with respect to Kraft seem proper and justified.
I do have some concerns regarding the cops allowing the trafficking to continue for a month to gain a critical mass of big name johns in the bag. That's a tough call, though. You have to balance the need to stop the trafficking with the need to have sufficient evidence to nail the traffickers. This concern is a bit tangential to Kraft, though, since he has no known responsibility for the trafficking that was likely going on amongst other women working there.