You are right but there is a morale issue to take into account. If someone is constantly coming in late it gets the back up of other workers, they will start to take advantage as they see a precedent being set etc.Employers would cope a lot better if they judged people on output and performance rather than number of hours spent in a set location. Obviously there's a balance to be struck, but I would be far more interested in whether or not someone is good and productive in their job than how long they're sat at their desk.