NRWTG is premised on the idea that grief is about the griever, not the grieve-ee. Grief is a coping mechanism for the person whose loved one has passed away, a way of dealing with the loss. Grief is thus relative to the griever -- the merits of, say, stoicism versus weeping being a function of the griever's psychology. This conception is coupled to the modern idea that it makes no sense to speak of right and wrong except in relations between people (morality is intrinsically intersubjective). Thus to the extent we can distinguish better from worse grieving, it's not a moral question, but rather a pragmatic one of what ways will help the griever to cope most effectively -- hence we speak of healthy vs unhealthy grief. But as pragmatic rather than moral concerns, they give no warrant for others to step in and interfere (rather than simply offering help and advice). At most, one's unhealthy grief may impair one's other obligations to other people.
The idea that grief is about the griever is also a modern idea. To the modern mind, there's nobody else who grief could be about. We generally hold that moral relationships can be established to actual other people, and possibly to potential other people (e.g. future generations), but not to former other people. The dead can't be harmed. The strictest version comes from a secular attitude -- the deceased no longer exists, as their personhood has evaporated. Modern religious views lead to similar conclusions -- the deceased is removed from us into heaven (or hell, or a newborn) and hence not available to be helped or harmed by how we grieve. A non-modern outlook, in contrast, may either hold that the deceased is still present, or that we can have obligations to those who can't be helped or harmed by our actions (e.g. to respect the memory and dignity of the deceased).
Another possible subject of grief is the family or community. Modern individualism (which underlies even less individualistic outlooks, since they simply expand the amount of effects on other individuals that are recognized), however, denies the possibility of having obligations to the family or community in terms of how one grieves. At most, we might recognize a particularly egregiously unusual form of grief as bringing embarassment-by-association on the other individual members of the group.
This is not to say that our way of thinking about death today is thoroughly modern -- consider, for example, the non-modern comfort we take in feeling of some action that the deceased "would have wanted it that way" (an instance in which the griever's non-modern preferences can justify certain forms of grief by the modern standard of "actually helps the griever cope"). But our conception of appropriate forms of grief on the part of others is governed by modern principles.