Why did he let Sky put a mike on him and, if he had no choice, why didn't he just get rid of it as soon as the meeting was over? He knows that Murdoch is out to get him and they would 'accidentally' leave it running for as long as they could get away with it.
They all have things like this, it is just that (at the moment) Clegg and Cameron haven't done anything as dumb as this.