Yes, but on an aircraft the captain is the dictator. They can do basically whatever they want within the confines of law and company policy - and honestly with enough seniority, which the captain on a transatlantic flight has a lot of - they can probably ignore company policy once or twice and get away with it and keep their job.
As far as I'm aware there is no law preventing the captain from deciding to go back because they don't like one of the passengers blasting their opinions to the entire aircraft. What the opinion is, its levels of subjectivity or objectivity, and whether or not it's popular is completely irrelevant.