Another company owns the redistribution rights for the UK and the AU, and that company has decided not to allow it on YouTube until a point in the future.
IIRC from other YouTube comments, there's a 2 week delay.
Kind of stupid in a global era, but that's what big corporations do.
The show is on in the UK tonight; from what I remember previously, the videos are opened up for the UK from that point. Not the worst thing in the world, restricting YouTube videos until they've had a chance to show the full episode in said country. It's not like this is just a clip - it's a whole segment of the show.
It just feels like what footballer fan (soccer you,yanks) would feel if they had to wait over 24 hours to watch a world cup match .... most would just find another way to watch it.
Other John Oliver videos are not restricted: http://polsy.org.uk/stuff/ytrestrict.cgi?ytid=uiN_-AEhTpk