In the case of Brandenburg v Ohio the US Supreme Court ruled that speech that encourages lawless action, and is also likely to be successful, is not protected by the First Amendment.
Donald Trump, in his effort to become president, has promised legal aid to supporters if they are arrested for attacking protesters. He has longed for the "good old days" where protesters were beaten. Trumps words have culminated in the assault of Rakeem Jones by Trump supporter John McGraw at a recent rally.
While some may question the motivations of an investigation, it is essential that those running for elected office aren't held to a lower standard than the rest of the country.