This clearance most likely occured at 9nm when it would have seemed "reasonable" at that point (usually you get "Landing Clearance" in CDG after checking into tower frequency and announcing which Runway you're lined up for).
If ATC cleared them at the typical point (1-2nm when the runway was actually clear) like every other European airport, it would have seemed quite odd to switch runways then and the UA crew would have probably questioned the clearance. Specific mistakes aside here, to me just stands as another reason that "clearing someone to land #3" is really poor practice, as much as you're 'used to it' in the US.
There is no encumbrance to ATC expedition by waiting to clear someone to land when the runway is actually clear... Clue's in the word isn't it?
I know I've had this precise debate with some of the above posters here before, but apart from arguing that they're used to it, I cannot see any safety advantage in issuing clearances like this. Hopefully this will be the thing that shakes up CDG to the standard of rest of the sensible world.