I think it's necessary to have a tradeoff - if someone from the tribal areas of Pakistan decides to kill his daughter, we want to be able to lock him up despite his belief that it's OK because of the society he comes from. After all, he did intend to kill her and we're pretty much agreed as a society that wanting to do that is a bad thing.
On the other hand there have been cases of teenagers asking another teenager for an ecstasy tablet and then dying, resulting in a conviction for the donor. That's a grey area - they know that Es are illegal but they have no intention of doing anybody any harm with them. You would think that the case could be made out for possession but not manslaughter. But we have judges who then get strongly criticised by the Appeal Court, so perhaps ignorantia juris non exculpat should also be applied to m'lud sometimes.