You are correct about the Bible not stating they were created with "original sin", but if Adam and Eve didn't know the difference between right and wrong before they ate of the tree of the knowledge of good and evil why were they so severely punished for eating of its fruit?

If you told your two-year old not to eat a cookie on the counter and the two-year old ate it anyway, would you cast the child out of the house and into the street?

And why did Yahweh place the "the one forbidden thing" in the garden, anyway?

The American mythologist, Joseph Campbell, has written about the occurence of "the one forbidden thing" in mythology.

Joseph Campbell notes that the Eden narrative's forbidden tree is an example of a motif "very popular in fairy tales, known to folklore students as the One Forbidden Thing". For another example of the One Forbidden Thing, see the Serbian fairy tale Bash Chelik, in which the hero is forbidden to open a certain door but he does anyway, thereby releasing the villain. Also see the classic story of Pandora's box, which existed in ancient Greek mythology.

There's an interview by Bill Moyers of the American mythologist Joseph Campbell in The Power of Myth in which Campbell discusses "the one forbidden thing."

MOYERS: What does it say about what all of us have in common that so many of these stories contain similar elements - the forbidden fruit, the woman? For example, these myths, these creation stories, contain a "thou shalt not." Man and woman rebel against that prohibition and move out on their own. After years and years of reading these things, I am still overwhelmed at the similarities in cultures that are far, far apart.

CAMPBELL: There is a standard folk tale motif called The One Forbidden Thing. Remember Bluebeard, who says to his wife "Don't open the closet"? And then one always disobeys. In the Old Testament story God points out the one forbidden thing. Now, God must have known very well that man was going to eat the forbidden fruit. But it was by doing that that man became the initiator of his own life. Life really began with that disobedience.

MOYERS: How do you explain these similarities?

CAMPBELL: There are two explanations. One explation is that the human psyche is essentially the same all over the world. The psyche is the inward experience of the human body, which is essentially the same in all human beings, with the same organs, the same instincts, the same impulses, the same conflicts, the same fears. Out of this common ground have come what Jung has called the archetypes, which are the common ideas of myths.

And Yahweh created the talking snake, but had no idea the talking snake would induce Eve to eat the fruit of the one forbidden thing? The talking snake in the mythology of Genesis was likely an element of the story derived from earlier Sumerian mythology.

Joseph Campbell believed that the serpent in the Eden story was lifted directly from either the Sumerian God Enki, God of Water and Wisdom, or his son Ningizzida. Both of them were identified as Serpent Gods, among other things. Enki was possessed of the food and water of life as well as the tablets of wisdom. Ningizzida was Lord of the Tree of Truth.

Reference: Asherah, Part II: The serpent’s bride.

Making the serpent a villain in the Jewish myth may simply have been a case of "the gods of the old religion become the devils of the new" as Margaret Murray, a British Egyptologist and anthropologist, is credited with saying.