The reason for this is the FAA PPL contains night flying as part of the training.
I have just got back from US and a student was actually cut loose on his solo(circuits) at night! Apparently allowed as long as adequate training/briefing has been given beforehand.
As FFF says though yuo will need the piece of paper in your hand before you can exercise the right. I think there is a XC requirement as well but if I remember rightly this can be undertaken as part of the 3 hours with the instructor and then you could spend the remaining 2 hours solo required doing touch-n-gos if thats what you felt like...