Is it ethical for professors to require that students maintain a balanced and disinterested tone in their papers and presentations?
At least based on the examples you gave, the rule you mention is clearly directed at teaching students to argue better and without resorting to various straw men, ad hominem attacks and other such rhetorical fallacies. Sure, there is some room for error or unfairness as with any policy, and depending on the precise way it is applied one can argue about whether this approach is effective in instilling good thinking habits among the students; but unethical it certainly isn't, as long as it is applied by the professor in good faith and out of a sincere belief that this serves the educational goals of the course.