Did PM actually receive a police caution or just a verbal warning (more likely) at the scene. The two are very different things and a caution can have an effect on eligibility for airside passes etc and will be recorded on the police national computer. For him to have received a caution it would have required arrest at the scene or an agreed time to return to the police station and be arrested by appointment followed by tape recorded interview.
I would suggest that if it was just a verbal warning then his behaviour, whilst possibly warranting police intervention, was not overly offensive and didn't cause undue harrassment, alarm or distress to members of the public. I wasn't there and I am in no way condoning his alleged behaviour and there are definately better ways of dealing with issues such as these.