Forbidden and Permitted Apples

At the center of the garden of Eden there were two trees (unless you think there was only one tree): the Tree of Life and the Tree of Knowledge of Good and Evil. The Torah relates to us that the serpent seduced Eve to eat from the Tree of Knowledge and she eventually fed the same fruit to Adam. As punishment (amongst others) the pair were exiled from the Garden lest they eat from the Tree of Life.

The two trees of the Garden make their appearance in two of Lewis’ fiction works: the Tree of Knowledge in Perelandra and the Tree of Life in The Magician's Nephew. As the Tree of Knowledge was at the root (pun intended) of the Original Sin, Perelandra recreates that scenario. The sin of eating from the Tree of Knowledge is replaced by the prohibition of sleeping overnight on Fixed Land. The Devil (the serpent), played by Weston, attempts to seduce Tinidril (Eve) to sin. The main thrust of the Devil’s argument is that in fact God wants Tinidril to disobey His command and act godly by demonstrating independence from Him. We discussed this here. 

The Tree of Life makes its appearance in The Magician's Nephew. There we find an actual garden surrounded by a gate with the Tree of Life at its center. The gate no doubt parallels the biblical cherubim and fiery sword used to guard the pathway to the Garden of Eden. However, unlike the biblical Tree of Life which appears to be impossible to get to, the Narnian Tree of Life is easily reachable. Actually, it seems anybody can come in and take from the fruit and, if one takes from the fruit for others, is even permitted to do so.

Why? 

Why does Lewis not adhere to the biblical story and essentially make the Tree of Life unreachable?

(To be continued when I figure out an answer. Feel free to make suggestions)