There was quite a bit of confusion about this in several threads. This is an older article but it explains very explicitly what happens.
http://features.engadget.com/entry/5170866986686287/
They use the camera for every one at the finish line for their finish order and have three cameras from different viewpoints and for backup data.
The transponders help sort out issues when there are crashes and mishaps in the last few kilometers as it gives them a record of where every one was at different points in time to help reconstruct events afterwards.