Is punishing someone ignorant of a law just?..... no.
I'm questioning God's expectations of us to follow him if we are isolated from civilization and religion in general.
Even though this is a tad out of bounds by strict forum rules, this much is at least civil and very likely genuine so I'll try to maybe shed some light.
I know I've posted relevant Scripture to this point in more than one place, and I think I did so in this thread. The point is NO ONE is w/o excuse! There is no such thing as "being ignorant of God's (basic) law."
This highlights the greatness of God Himself, and His ability to "write His law in our hearts," put His word in our mouth, etc. This is possible to all mankind equally only because of the shed blood of Jesus Christ, and I see the modern Church mistakenly limiting God's greatness on this issue.
Don't get me wrong, the preaching of the Gospel, sound teaching, and the whole process of Discipling is still very important. The only real way to be sure we're "hearing God" and later to learn to recognize His voice (and to distinguish it from our own thoughts, etc) is by comparing it to His
written word.
Even so, the more basic things can be known from observing our physical world, every detail of which "speaks" of God, in it's own way. So we are ALL w/o excuse.
If any of this is unclear please ask, as there are myriads of ways of getting the same point across. God works like that! "We are the sheep of His pasture" means (among other things) he knows even the smartest of us are downright DUMB compared to Him

He kindly illustrates important points not only over and over again, but in various ways in Scripture. The point being, sooner or later we'll get it if we sincerely want to know Him. I do wish you success in that venture ...