Really? How about in the case of armed robbery? Or murder? Can they choose not to arrest? Yes, but there will be serious consequences. BTW I voted for it too.
If a single officer is the only witness to a murder, what consequences will there be if the officer chooses not to make an arrest? Yes, there's nuance and as soon as multiple people are involved there's the risk of public outcry, but as far as I know, there's not a legal obligation for a peace officer to make an arrest just because they have witnessed a crime. Whether there is a social obligation to make an arrest is highly situational and depends on not only the crime but the surrounding circumstances.