The owners created the venue but the members sustain it - even it is "free". It's impossible to monitor this site 24/7 without a paid staff doing so. The business model of being a "free" site probably doesn't justify paying a staff to monitor the site 24/7. I'd say most other similar sites don't monitor theirs 24/7 either.
That being said, decisions to ban a member should not be based entirely on one incident - unless it's blatantly egregious. That members history should be taken into account and, and based on the incident, dealt with in a fair manner according to rules set forth by the owners. There have been obvious exceptions, especially in the last few months, of trolls and extremists inciting the masses. Those should be, and have been (for the most part) , dealt with accordingly. But to arbitrarily ban a long time member for one incident with a history of making significant contributions is not justifiable.