It depends on policy of the venue and artist and can vary. For example, a lot of concerts at SAP Arena are literally called "Zero Enforcement" where ushers are

It depends on policy of the venue and artist and can vary. For example, a lot of concerts at SAP Arena are literally called "Zero Enforcement" where ushers are disallowed to intervene in any inter-patron situation outside of medical emergency. It ends up being a Lord of the Flies situation where the fittest do (or stop) what they want.