I'm not the person you were replying to. I'm only defending the common carrier concept, not everything he wrote. But I guess I can explain the difference between banning specific communities that are not violating American law and, on the other hand, American laws that do not imply endorsement.
America's obscenity laws have specifically been tweaked to reflect the Supreme Court ruling that the federal government cannot draw the line between work with redeeming artistic value and work without redeeming artistic value, because the government is not in the business of endorsing some photos and film as art and rejecting others as trash. However, the federal government can ban child pornography because producing it involves the rape of children (which is a different question from assessing the artistic value of the media). So as a fundamental doctrine of American law, the government does not and cannot take any position on the artistic value of illegal, obscene material; it needs other grounds for a prohibition. Thus there is no implied rejection of the content expressed in the banned material, nor any endorsement of what is in the tolerated material. Likewise, when Reddit complies with federal law but declines to ban speech permitted under federal law, there is no implied rejection or endorsement.
Likewise (do I need to point this out?) the law on murder is not a law that is designed to protect any specific group from, or leave any specific group vulnerable to, murder. The law is justified by a public interested in protecting the lives and liberties of American subjects, with zero intended rejection or endorsement of what specific Americans do with their lives. The law would be an endorsement if it specifically protected certain groups from murder, while leaving other groups undefended, thus implying the government approves of the former sorts of lives and rejects the latter, but it doesn't so your point is unclear.