Parler’s content guidelines appear to be written to make it misleadingly look like wholly discretionary aspects of their moderation policy are legally required. Like most platforms they plan to “censor” lots of First Amendment protected content. http://legal.parler.com/documents/guidelines.pdf

They bar “fighting words” (with a link to a Supreme Court ruling), a doctrine that’s been sharply narrowed and in any event only applies to in-person interactions with a direct threat of violence.

They bar “indecency” with a link to the FCC definition—but FCC regulation of “indecency” applies to broadcasts over the public airwaves. There’s nothing illegal about posting “indecent” content on the Internet.

They bar spam and unsolicited advertising, which does indeed have diminished First Amendment protection, but which is not illegal as such: Parler could allow it, but has decided they don’t wish to.

They ban “pornography” with a link to Roth v. United States, but no mention of later decisions more protective of explicit speech, including Reno vs. ACLU, which specifically struck down a regulation of Internet pornography on First Amendment grounds. https://www.law.cornell.edu/supremecourt/text/521/844

Unsurprisingly, guidelines notwithstanding, they reserve the right to "remove any content and terminate your access to the Services at any time and for any reason or no reason.” https://legal.parler.com/documents/useragreement.pdf

In short, Parler is trying awfully hard to make it look like their policy is to permit the absolute maximum of speech that’s legally permissible. But of course, that’s not true at all. They are choosing to restrict large categories of lawful speech, as they have every right to.

They’re doing this because almost nobody would actually want to use a platform where “the only moderation policy is the First Amendment.” *Pretending* to be such a platform is great marketing, but actually *being* it would send users screaming back to Twitter in about 2 minutes.

They COULD, of course, just say: “If you don’t like Twitter or Facebook’s content moderation policy, we have chosen a different one you may prefer.” But that might prompt the awkward question: "Then why should government enforce one market preference on other platforms?"