Let's say that quiz interactions from McMahon come out to 5% of ad impressions for all of BF. What would a site as big as BF pay to ensure that those 5% of impressions don't disappear overnight? At most companies, I'd imagine quite a bit.
Even if we look at the low number for revenue generated from McMahon's quizzes ($200k), it's easy to imagine a company paying that much or more to continue driving those numbers. If someone is providing you a service for free that accounts for a very meaningful percentage of revenue, how much revenue could they be generating for you if you actually incentivized them to do that work? 1.25x? 1.5x? 2x? More?
It would seem to me that BF never had the realization that "huh, maybe we should protect this asset instead of protecting our short-term profits and praying she keeps doing this for free forever."