I would guess that a bonus to Enchant Weapon might cause all sorts of problems if the item is worn but then later removed.
The game would then see the impossible situation of items with more Charms than they can actually have, which may well cause some kind of internal logic fault and cause the game to crash.
That's why they are not in the game.
That's only a guess, mind <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
