"Core D&D Rules" doesn't make sense as a difficulty level.
I would agree, except "Core D&D Rules" was a difficulty level in the original BG 1 & 2 (one step up from Standard difficulty), so it could make sense to implement it for players who like the challenge of playing D&D RAW.
Also, I'm sure there are players out there who enjoy being able to dip, backstab and throw barrels, and I don't want to take away their fun, even if it kinda spoils the experience for me.
