He got arrested when he didnt comply with the order. As already said theres no point having powers and then not enforcing them.
Disclaimer: I didnt see it, so could be speaking out of my ass!
But if i was told to stop doing something which wasnt breaking a law, and ORDERED to leave the area by a police officer, i'd be inclined to ignore it to if i wanted to. Sober or otherwise.
The police have no greater power than the man on the street, other than the power of arrest. If they choose to enforce that power they need a significant reason - and if the action didnt justify it they'd have to explain it away. The role of police is to protect the public, ordering people away from an area doesn't fit with their power or the public protection, unless the person is affecting other peoples safety - at which point they should be arrested.