This is how it's been explained to me in the past as well. Hence, the reason they don't include walks and HRs. However, is this saying that pitchers don't have control over someone hitting a single, double, or triple off of them? It's like the stat is saying that pitchers are responsible for walks and HR, but not that double that get's hit high off the OF wall. The pitcher has some control, but not really that much on what really are somewhat rare events. And part of this is selection bias, if a pitcher keeps giving up hard hit balls over 3 starts he's not going to see start #4. So when we are looking at major league talent the pitchers only have some control over slugging against. My main point however is that what has been called "good pitching" has been more good defense and good luck. You can't count on luck going forward and Weeks' injury harms the defense.