First of all regarding "smashing car windows".

You are of course right that a single occasion would not, most likely, result in action.

Now let's say that the following night a different witness came forward saying he had seen me smash another car window - then a few nights later someone else came forward and said they'd seen me smash yet another car window.  

Still no direct evidence in any of the cases.  Each case when looked at individually depends on a, "he said, she said" scenario.  The similarity between the cases however and the independence of the witness evidence would almost certainly result in me being charged with all three offences and most likely being found guilty in court - rightly so.

In the second scenario you paint I can relate to exactly what you are saying - what you are describing though is not a problem with the accusation being investigated - I would expect every such situation to be thoroughly looked into.  The problem doesn't lie with the police, (they very rarely release names or details of any accusation).  It is the media, social networks and individuals who are the root of this particular problem.

The accused in such cases do suffer badly even when innocent of any ill doing - how you get around this depends on an unlikely change in social behaviour.  That however doesn't mean that any accusation of assault should be ignored, or is that what you are suggesting?