On: 4.2.2.2: The Story Behind a DNS Legend

[ck2]

Just imagine how much traffic they get.

If they really didn't want people to use it, I am sure it would be easy for them to block whole swaths of the net from using it.

[orofino → ck2]

They mean not to use it as your primary DNS server. Pinging it and using it for temporary DNS is likely fine, I wouldn't suggest statically assigning it for permanent use though.