Where did I say violence is acceptable, @TizerorFizz? Unstructured play does not mean no boundaries or rules, it means child-directed play. The huge benefit comes from children learning and discovering the boundaries themselves in a way they don't in structured scenarios like sport. They can let off steam and follow their own interests, push themselves and run a little wild, but no, unstructured play does not mean a violent free-for-all.
Behaviour would dramatically improve if parents and schools committed to giving children a certain number of hours of active, unstructured, group play a week, instead of too often cutting playtime short. But schools don't have the resources or time and parents are often stretched for time too, and what's the result? A pressure-cooker situation with limited opportunities to let off steam, combined with insufficiently-enforced boundaries, which means kids go feral whenever they get the opportunity.
Useless to talk about discipline if children's needs aren't first being met. It's only ever going to be crowd control in that situation.
And before anyone accuses me of being a 'wet lettuce" parent, I am extremely good at crowd control. I have run and helped with enough school trips, parties and playdates that I know exactly how to get small children to comply will instructions. My own child knows that if he crosses the line, I will squish him without compunction.