ICAO introduced Reduced Runway Separation Procedures a couple of years ago which permit a landing a/c to cross the threshold when a preceding a/c is at least 2400m down the runway and in motion towards an exit. VMC by day only, as someone said. Many ANSPs allow their Tower Controllers to use their judgment that this condition will exist when the following a/c is on short final to avoid having to give a late landing clearance.
I have to say I'm with HD on the logic used by the FAA. Why not make the default that everyone is cleared to land unless told otherwise, if you're going to clear number 5 to land at 15 miles?