In asking this question, I am not referring to exceptional cases like Yovani Gallardo, Madison Bumgarner, or Jake Arrieta, who hit about as well as position players. But I am referring to "most" pitchers, whose batting averages are in the .100s, about half as high as average position players.
I read somewhere that there is a theoretical batting average below which it was right to always throw strikes because the chances of getting the person out was so high that it was worthwhile not to give him a chance to walk, and as a practical matter, there was a somewhat higher threshold below which it made sense to throw mostly strikes, thereby giving the batter a minimal chance to walk.
My best recollection is that the rule is that one should "never" walk a batter (incurring a sure loss) whose average was below .075 even if he all home runs, and that it made sense to never walk a batter with an average as high as .225 if he never made extra base hits.
Don't most National League pitchers fall beneath these thresholds as batters? (Or if I have stated them wrongly, the "corrected" ones?) So why do pitchers often throw enough balls to walk them, when they could get them out by throwing mostly strikes?
Put another way, why does practice differ from theory in this instance?