In the example, the parents picking up their kids on time beforehand were doing so for altruistic reasons alone. When the choice became framed economically, parents started picking up their kids on time less often.

You say they need to raise the fine, but the point here is that it's not economically feasible to raise the reward for running a Tor node indefinitely. The max amount paided out for running a node may be less of an incentive than the existing prosocial incentive to do so.