There's a difference between legality and ethics.
Ethics to you does not mean ethics to me.
Many other American politicians were behind similar riots and get away with it. For example, the riots surrounding the nomination and confirmation of Justice Kavanaugh where the Supreme Court building was stormed.
Many of his followers still believe he will be reinstated as POTUS, allowing him the bullhorn of his Twitter account would be unethical as it would make it easier to galvanize these folks into action again.
That's quite ridiculous. Twitter was more than happy to back open rebellion in other countries, such as those in the Middle East. It comes down to a matter of what is Twitter (and Facebook). Are they just platforms, or are they in the business of publihing certain opinions.