I think that NIP is related with the issue of modestly. What does it mean to be modest? I think that at it's heart, it is about not purposefully trying to sexually titillate people. And in different places and cultures, different parts of the body are considered erotic; hair, ankles, breasts, etc.
To some degree I think the differences are really random, and we have to recognize that. If I go around wearing something inappropriate to arouse the public at large, it's immodest. If I do it to anger them, it's probably unkind. If I don't realize I shouldn't, then at worst, I am being rude by not paying attention. Every culture, even those that wear almost nothing, has behaviors that are not acceptable. There is always behavior that is going to offend people. And a person can be immodest even when fully dressed.
NIP is difficult in NA, I think, partly because we are in a place where practices and attitudes have been changing. So that there are people who have been raise/ are used to different definitions of modesty. As a society we have decided that NIP is ok and to be protected, but it will take a while for it to permeate the culture. And breasts ARE still considered a largely private part of the body here. I don't really think that that is either right or wrong, in itself.
So to my mind, as long as it is possible to NIP, I am largely willing to go along with the local culture, whether that means being discrete
in so far as is practical and good for the baby or letting it all hang out. I don't think it is possible to get away from the cultural element. And perhaps we should be kind to those for whom this represents a change in thinking.
But North America is so weird, you can be practically naked to sell underwear, not to nurse your baby.

: