Trump’s appointees to the Supreme Court craft the new rule.
They say that since the News Media are the “enemies of the people”, the people questioned by reporters deserve protection from oppressive questions.
That means that reporters' questions have the force of an arrest: Trump is not free to leave until he's satisfactorily answered the questions, or until he's charged. Even then, he can be detained, pending a judgment by the court of public opinion.
Actually, we've already had a steady drumbeat of guilty verdicts there. On November 8, 2016, the court ruled against him by a margin of almost three million votes, which in a functioning system would have been enforced, preventing him from occupying the Oval Office. Appeal after appeal has been fruitless. Nationwide approval polls consistently find a majority disapproving of him. Most recently, the August 19-21 State TV Fox News found him guilty by a vote of 53%-45% (approximately 535 to 454).