Strictly speaking, they’re leveraging free users to increase the number of domains they have under their DNS service. This gives them a larger end-user reach, as it in turn makes ISPs hit their DNS servers more frequently. The increased usage better positions them to lead peering agreement discussions with ISPs. More peering agreements leads to overall cheaper bandwidth for their CDN and faster responses, which they can use as a selling point for their enterprise clients. The benefits are pretty universal, so is actually a good thing for everyone all around… that is unless you’re trying to become a competitor and get your own peering agreement setup, as it’d be quite a bit harder for you to acquire customers at the same scale/pace.