MurderBay technically already exists in the form of Craigslist, Backpage, and other more local classified posting sites. Whether they be jokes or serious offers/requests, postings for murder, drugs, human-trafficking, and prostitution occur hourly on legal websites, many hosted within the US. Backpage in particular has been targeted by human rights groups as it has had multiple cases where human trafficking and child pornography were arranged/traded through the site. The website, to my knowledge, was compliant with law enforcement to the degree required by law, and the sites still exist to this day.
The reason is that, like with other websites that collect user submitted content, as long as the operators are compliant with the law in regards to the content, they're not liable in the way most people think.
So no, unless MurderBay is seeking out assassins and murders, you should not expect law enforcement to take it down. You probably can expect quite a few phonecalls and emails though. Likewise, unless ThePirateBay is seeking out pirated content and pirate groups, it should not be expected that it be taken down.
[1]probably still does; I can't access it through regular web browsing due to a legal ruling...
I get it: you think sentiment should be conclusive, and in many cases you could say that it is in practice, but it's hard if not impossible to make a case against a disfavorable entity solely on that basis.
And the RIAA has certainly disparaged defendants in many if not all cases. Hard to say how much effect it really has, though.
A better analogy IMO would be a website that gave you directions to get information as to where there was a gap that you could use to access a private beach, both are giving information that allows you to later commit a tort. Such a website would be any search engine.
As Far As I Association of America?