Oneyedvic is absolutely right - the phrase 'not in the public interest', although somewhat inflamatory at face value, is an accurate decription of the decision.
In cases where this is the result, it simply means that the Crown prosecution Service are not willing to commit tax-payers' money to fund a case which they, the CPS, do not feel they have a better than even chance of winning.
In that instance, the 'public interest' is served, but it does appear harsh in circumstances like these, when the 'public interest' appears not to have been served at all.
Fortunately, in these cases, some seriously planet-sized brains examine evidence with a detached viewpoint and consider the likely outcome of a trial, with every aspect considered, and make an informed unemotional decision which is judged to be the best in the circumstances.
It is thanks to this system that we have judges and juries, not lynch mobs and vigilantes.