What happens if you accidentally install NTP publicly, creating a public DDoS reflector? When it gets used, you get emails from people who are getting DDoSed by your IP, saying "hey, some attacker is using your system to compromise me." Now there are selfish and altruistic motives, both, invoked by that feedback, causing you to ban inbound NTP requests. Even if your server responsiveness doesn't matter much to you, you'd still like to limit legal liability; even if legal liability doesn't matter much to you; you still feel offended that someone else is twisting your systems to nefarious purpose.
So one nuance is that SAV doesn't really offer any feedback for the network operators: nobody from the outside ever complains so nobody from the inside ever hears about the problem. Maybe altruism is only economically viable with a good feedback mechanism.