The recommendation engine doesn't know what the content is or in what way it is similar, it just knows that people who view this video often view this other video. On the bright side the recommendation engine grouping all these videos means that once YouTube identifies one video as inappropriate they can easily use the recommendations to find and delete all the other videos that are also being viewed by the same pedos.

Some of the videos were up for months, and youtube did nothing to remove them after being asked.

The "recommendation engine" is a stupid "people searching for x clicked on y" thing. Not intelligent at all.

And banning keywords won't help, that's just whack-a-mole'ing.