Because for almost any other site this would be seen as a net negative. In this case, we've all (or very nearly all) benefited from stackoverflow in a meaningfu

Because for almost any other site this would be seen as a net negative. In this case, we've all (or very nearly all) benefited from stackoverflow in a meaningful and tangible (often monetary) way. The preemptive defense will hopefully reduce any knee-jerk criticism over ads.