Platforms such as Twitter, Faceberg, and YouTube do not have to censor contributors. They are literally protected by law to allow people to post whatever the fuck they want.
https://www.eff.org/issues/cda230
Section 230 says that "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" (47 U.S.C. § 230). In other words, online intermediaries that host or republish speech are protected against a range of laws that might otherwise be used to hold them legally responsible for what others say and do. The protected intermediaries include not only regular Internet Service Providers (ISPs), but also a range of "interactive computer service providers," including basically any online service that publishes third-party content. Though there are important exceptions for certain criminal and intellectual property-based claims, CDA 230 creates a broad protection that has allowed innovation and free speech online to flourish