What people allege they saw and what the police find upon looking into a complaint are two different things.
The article in the Daily Mail states it was people from a nearby building that complained. How far away were they? Were there any complaints from anyone who was nearby with a clear line of sight?
Notice no mention is made of the police questioning people in the area to see if the couple had been having sex in public.
Notice too that the police themselves state that no crime had been committed. Which means they had every reason to believe the couple were not having sex.
True on all you said BUT IF there was a call they had to follow up on it. HAD this little twat went ahead, showed them her id (since she was a nobody to them) and not started off with the race card, everything would have been fine.