Because nice guy they probably couldnt..i dont know the case at all. but there is every possibility that who ever charged or investigated felt very similar to how most people would feel reading the news article.
I also suspect that due to the fact that this is a DV case and therefore contains someone very vulnerable the police, CPS and courts would have very strict legally binding guidelines that would have to be followed, completely removing any power of discretion.
for example for most offences there is no specific power of arrest, there is a general power which means that on each an every occasion the officer has to justify why That arrest was necessary. However by local guidelines rather than law, officers are required to justify why the didnt arrest in DV cases where a power to do so existed. The reason for this is to remove the ability of discretion, officers getting it wrong and someone being seriously assaulted or worse. if you turn up, dont arrest (or take other positive action) and then offender goes onto murder etc victim when you had been directed to do so then not only are you morally responsible for that persons death then you are likely to be legally as well. i suspect most people would not chose to use their discretion in that scenario and follow set protocols.
there are, unfortunately, many violent relationships (where both parties are violent and abusive towards each other) and there fewer but still far too many where one is completely subjugated by the other. i may be wrong but often feel that the current law is built around preventing the violence from the former kind and woefully inadequate at dealing with the issues around the later. it appears relatively straight forward to write a law and direction about how to deal with two violent and abusive individuals. but where you have circumstances that arise out of individual relationships i.e a person is put under duress in a specific manner and they react in a further specific manner it must be nearly impossible to write a law that acts as a catch all for all the possible circumstances. Those issues can only be changed by case law...such as the one that's going through above.
perhaps victims, which in most cases are women, should have more of a voice in writting these laws in the first place. a law has to be hard to change to stop corruption which is why it needs to be a case of get it right first time. its horrible and disgusting that a victim has to be dragged through the process to make these changes. is there another way though?