This is just an anecdote, but I've heard that even if some professors strongly suspect cheating, since the system involves so much red tape and leaves the professor with little power, it is simply not worth pursuing it and ending up losing upwards of half a day. This means, professors, whose free time is already perilously low have all the incentives in the world not to pursue students who cheat, which only makes it that much easier for them to get away with it.