Yeah that makes sense to me. If you come up to me and say “you have to arrest that guy; he’s stealing from me” I have to do a lot of research to make sure that everything is correct.

On the other hand, if I see you steal from me, I don’t have to do a lot of research. I am a first party to the thing. I can be sure.

It’s the difference between a policeman arriving on the scene of an assault and someone actually assaulting the policeman.

The acting party being the affected party simplifies things because you know you’re not a “confused deputy”.