Perhaps it is just to avoid scaring investors, going by Larry's concern assuaging in the second paragraph of his post.
One of the Google founders was also behind funding the first purely lab-grown burger eaten in London a few months ago. That also had an obviously professional PR campaign attached to it, although little of this was explicit in the press. A professionally edited HD recording of the event appeared on the BBC web site only a few minutes after publication, so obviously some PR mechanism was at play.
But that event wasn't to promote Google or anything else, it was absent of any commercial labels attached to the copy or the video itself, and no mention was made of who organized or funded it. I have no idea what the message, or the point of that campaign was either.
What are these campaigns trying to tell us?