Protection from civil liability is a privilege granted by government, it should be afforded to those companies in exchange for public service - guaranteed freedom of speech and expression, they should all be truly free and open platforms for the public. Many forms of expression, including calls for violence, are already not protected by the first amendment, so I think it's a perfectly sufficient metric for what is and is not acceptable. If it's illegal IRL, it should be illegal online, and vice versa - if it's legal IRL then it should be perfectly acceptable online.