It isn't though. I did a lot of research on this back in maybe 2010-11...posted it in the forums thought it's probably gone. baseball-reference.com shows B vs P matchup data (though I can't find it right now) so I looked for places where a very low BB pitcher faced a very high BB hitter, with at least 100 PAs. While I don't remember the exact results, the average number of walks in those PAs was much closer to the hitter's rate than the pitcher's. The WIS data is drawn from real life, not from some theoretical estimate. The batter is a much stronger predictor of whether a PA will end up in a BB than the pitcher. And this is true even for extreme control pitchers.