I'm fairly sure that the "4.2.2.x was never meant to be public" line is a myth. Though the NANOG thread cited in that post is good historical background, it is contradicted by more modern sources:
"[...] DNS infrastructure is largely split into two types; open (public) and closed (private). Open DNS is provided by companies like OpenDNS, Google and Level 3. You can use it wherever you are on the Internet with no restrictions or authentication required."
- Mark Taylor, VP at Level3
http://blog.level3.com/level-3-network/a-flawed-study-of-cdn...
I can't find any cite where anyone else who I would consider a reliable source in the DNS world (Vixie, &c) repeat this claim. To the contrary, Level 3 is often grouped with Google, OpenDNS and others in discussions of open public resolvers [1][2][3], and those in the know never seem to speak up and say otherwise in these discussions.
That being said, I have absolutely no personal knowledge on any of this.
[1] http://www.maawg.org/system/files/Fergie_DNS_Open_Resolver_M...
[2] http://markmail.org/message/gh7f2wvfbn5mpvuq
[3] http://www.circleid.com/posts/87143_dns_not_a_guessing_game/...
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.
Google's motivations are long-range, but simple - more ad dollars. Faster DNS means more people using the web (rather than give up in disgust - and if don't believe that happens, let me introduce you to comcasts's DNS servers...); more people using the web means more page view which means more ad revenue.
So no, definitely not altruistic.
I just thought it was to track user actions via DNS - they can see which sites you visit without needing tracking bugs on those sites. Better profiling means better ad serving for Google ... profit.
They specifically promise not to do anything like that.
https://developers.google.com/speed/public-dns/privacy?hl=en
"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."