Miss out on what ad revenue? Advertising porn in AdWords is against terms. The point is, if competitor sites come up in the safe search and yours doesn't, you lose those visitors. Users explicitly need to turn off safe search to see your results. They won't do that if they find competitor sites within the normal results. This means no site owner will do that, regardless if it helps work places or schools filter it better. They don't care about that, they care about visitors.