Timothy:
Complicated question. Someone could dedicate their entire practice to first amendment cases related to this matter. That much said, here's some thoughts and results of some research. Please do not rely on what I am writing as I am not your lawyer; please consult legal counsel. Anything I write you should also include here as applied to me to the widest extent possible all the language of the disclaimer at the bottom of the Muddy Waters release on CCME. ;)
Under the Communications Decency Act in 1996, Section 230 no provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider and there is even immunity for the publisher: "[n]o cause of action may be brought and no liability may be imposed under any State or local law that is inconsistent with this section.
Section 230 excepts federal criminal liability and intellectual property law. 47 U.S.C. §§ 230(e)(1) (criminal) and (e)(2) (intellectual property); see also Gucci America, Inc. v. Hall & Associates, 135 F. Supp. 2d 409 (S.D.N.Y. 2001) (no immunity for contributory liability for trademark infringement). In Perfect 10, Inc. v. CCBill LLC, 481 F.3d 751 (9th Cir. Mar. 29, 2007; amended opinion issued May 31, 2007) the Court of Appeals ruled that the exception for intellectual property law applies only to federal intellectual property law, reversing a district court ruling that the exception applies to state right of publicity claims. Cf. Carfano, 339 F.3d 1119 (dismissing, inter alia, right of publicity claim under Section 230 without discussion).
The criminal exception seems to be for the government, and not fro private claims. Under Section 230 of the Communications Decency Act, a magistrate judge for the U.S. District Court for the Eastern District of Texas concluded in a report and recommendation issued Jan. 18 (Doe v. Bates, E.D. Texas, No. 5:05cv91, 1/18/06). Though Section 230 includes an exception from immunity for violations of federal criminal statutes, that exception applies to federal enforcement of those statutes--not to private civil claims permitted under them.
Also if SA starts removing content, they become more like a publisher and less like a distributer. They do offer a process by which to remove content so this does suggest they have assumed a degree of editorial participation.
Seems you have a good faith basis to at least explain to them they may be accountable for their knowing participation in such an obvious attempt to publish and then not remove forged bogus documents that have great influence on the markets for the benefit of short sellers in violation of the Securities Act.
Rattle away and see where it leads.
-Andrew