I think the crux of the policy is that staff have been told that they will be supported by their management if they decide to offload passengers due to unruly behaviour, without fear of being reprimanded for making the flight late. I'm not a total expert but could this be because of the relatively low number of rotations their aircraft do each day meaning lengthly delays for other flights won't be a knock on effect?
The Easyjets of the world may not be as supportive to their staff if they make the 0600hrs departure delayed due to an offload as per my post above. The two we had to suffer a flight with were prime candidates for being dragged off.