Two items lifted from the comments: 

When the public have to do what the police apparently won’t.

Our betters at large.

Imagine being so self-absorbed and self-flattering, so untroubled by normal boundaries, that you don’t anticipate how your own disruptive behaviour will tend to be viewed by the wider public – the people on whom your behaviour is being inflicted. A wider public that for the most part can’t afford to spend days on end indulging in Student Union theatrics.

Update: 

In the comments over at Samizdata, Mike Solent adds, 

This is an interesting example of public order being served by the absence of the police rather than its presence.

Well, yes. Quite. 

Update 2:

Know your place.