I assume species that evolve on different planets would be independent from each other and have their own policies. I do not know why a ‘no contact’ policy would have such importance to advanced species to the point where they are willing to resort to threats or violence to enforce them. Not only that but different departments within those species may have different policies (the same way that Denmark and Cuba have different foreign policies but are both within the human race).
The concept that all alien races would reach the same agreement on foreign policy makes little sense to me when humans are incapable of that kind of cohesiveness and unity. Then again maybe this is the case and if so maybe UFOs are like guerrilla fighters and an attempt at making their presence known w/o alerting the authorities (either theirs or ours).
What may also be true is that the concept of a multiverse, parallel universes, other dimensions, etc. is true and as a result there are virtually infinite intelligent species that exist and can be contacted. In that case the human race is probably pretty mundane and boring. It would be like a solitary fly somewhere in Spain wondering why the human race hasn’t contacted it. There are infinitely more interesting and important races to contact.