In this case, the post is about the company's revenue stream (or lack thereof), as it pertains to money-for-better-reviews. If Yelp's claim is false, and they are mounting a marketing campaign to knowingly deceive shareholders and potential shareholders about revenue sources, then yes it would be a criminal act.
In fact, digging around a little reveals there has been a class-action lawsuit for a jury trial on these very grounds. [1] Granted, they sited Yelp's 10-K filing with the SEC, but I seriously doubt a jury would say "well, it wasn't on their 10-K, so the blog post is fine."
That's a banal observation, I know. But I was moved to comment when you suggested that they'd face criminal penalties stemming again from their being public. No, that seems very dubious.
I did not say -- nor do I believe -- that the mere possibility of criminal charges in virtue of Yelp's public status "forecloses on any opportunity" for criminal activity at the company. Rather, I invited foobarcrunch to "at least consider that Yelp is a publicly traded company". While the possibility of criminal charges obviously do not prevent all white-collar crime, it is certainly a deterrent in many cases. In isolation, this neither proves nor disproves that Yelp is acting inappropriately. However, I think it has some merit as part of one's personal, cumulative assessment of whether Yelp is engaged in extortion.
Second, in my previous posts I said "criminal offense" and "criminal act". You've taken this to mean "they'd face criminal penalties". This is not what I suggested. I fully recognize that not all criminal acts are prosecuted. At most, we can say that criminal acts carry with them the possibility of criminal charges. For many, this bare possibility is an effective deterrent (see paragraph above).
Again, I'm not trying to beat up on you here. Just be careful that you don't inadvertently respond to straw-man arguments when you discuss the issues.