the police have every right to physically defect vehicles on the spot unless it is in a private area.... and if they really want to get you & if they are able to identify your vehicle by the number plates, you will most likely be receiving a letter requesting for an inspection.
from the pix i have seen from this BBT night event, it was an open area, and looks like everyone can just walk in to admir/laugh at the cars... so what makes it different for the cops to walk in and having a power trip?
during my auto salon competing days, i have seen cops walking inside the auto salon show arenas/halls/exhibition centres taking down number plates and defect cars (during the weekend, while competing/showing), as well as pulling cars over defecting when they have lfet the show on sunday evening/night.