And finally, and back to my original point, what we say online can be actionable (get you sued.) We are NOT anonymous.
Libel Online - Lawyers.com
"You Aren't Anonymous
Let's get this straight. Web sites track IP addresses. Web sites will give up IP addresses to law enforcement and to subpoenas issued in civil cases. Your internet service provider will provide account details of IP addresses under the same circumstances. So what you say on the internet easily can be tracked to you.
The Internet Didn't Repeal the Law of Defamation
Yes, you can be held liable for defamatory things you say on the internet. It happens all the time now. Many examples come from negative feedback left by parties to transactions on internet auction sites, like eBay. In one case, a Florida attorney sued a dissatisfied buyer for libel. The buyer left negative feedback on eBay, saying the attorney was a "bad seller, he has the ethics of a used car salesman." The attorney sued the buyer for defamation of character.
The internet differs somewhat from traditional mainstream media when it comes to defamation and related lawsuits. The Communications Decency Act gives immunity to internet service providers for comments posted by their users. This immunity extends as well to web sites that permit user comments. If the web site doesn't prompt the illegal comments or behavior, it will not be liable for it. Of course, this doesn't protect the person leaving the offending comment. "
In other words, and again, on-point, while usmb may not be held liable for posts of its users, if your users keep getting hauled into court because you allow defamation that might have negative consequences for you when it comes to selling ads. So forbidding certain things is in your business-model best interest.