Presumably it takes non-0 costs to maintain 4.2.2.x DNS servers. While I'd want to believe Google and L3 just try to help out the public at large with free DNS services, I suspect they are not doing that just purely out of altruism.
So no, definitely not altruistic.
"We built Google Public DNS to make the web faster and to retain as little information about usage as we could, while still being able to detect and fix problems. Google Public DNS does not permanently store personally identifiable information."