I read the OP as suggesting that broadly speaking, when a monogamous relationship ends, the social expectation is that the ex-partners don't have much to do with each other. (An exception is co-parenting of course.) It is surprising to many that one would stay in contact with an ex, much less be friendly or retain a deep connection. For instance, Beaker's sister was quite surprised that we still talk regularly. It's been my experience that this is a common attitude. And people don't follow the broad social expectation in their own lives, but that expectation is still around.
I also think this is another in the OP's quest to get folks to be more conscious about their relationships. After all if you've taken the time and made the effort to be polyamorous, then there is the opportunity to examine old ideas and keep or jettison as you see fit. It's not that poly is more evolved but that it can open the door to looking at and re-examining social programming and assumptions. Of course mono folks can do this too. Poly is not required for this examination and weighing of social mores. But it's been my experience that people who move outside the mainstream in whatever way are more likely to do this self work than folks who are more mainstream. There is nothing wrong with being more mainstream but it is a place of privilege - and one of those privileges is not having to think about social mores if one doesn't want to.