Via BoingBoing:

It’s clear to me that kettling is punitive, not preventative. It isn’t intended to cool out a dangerous situation (if that were the case, police would release demonstrators in a small, steady dribble, defusing whatever chaos they’re trying to prevent). It’s intended to punish protestors for democratically assembling in public, and to frighten off potential supporters who would like to express their displeasure with government