If you think ethics are limited to 'the law' or some narrow 'as long as you follow the written rules' behavioural norm, you don't understand ethics or ethical behaviour. Ethics are values-based. They are moral principles of behaviour. And perception of propriety matters.
This is precisely why the current PMO and its defenders seem to keep running into the same problem. They're vaguely conscious that they're not breaking some written rule, and no one bothers to ask 'Is this principled?' or 'How will this look?' Those conditioning thoughts matter.
A secondary institutional problem might be that our 'Ethics Commissioner' actually has a narrow 'conflict of interest' mandate that reinforces the incorrect view. Few seem to understand you can follow CoI rules and still be acting improperly on a moral, ethical standard.
Which isn't to say the ethics commish *won't* find wrongdoing in this 3rd(!) investigation of the PM. But it's almost irrelevant to the broader ethical question of whether the PM should have recused himself from the decision or whether the sole sourcing was obviously unjustified.