Anyone capable and wiling of exposing their life thorugh a one-hour TV special puts themself in a situation to be criticized.... surely LeBron knew what he was getting himself into. And, as a public figure who escalated his stature as such through the TV special, proving slander or libel becomes very difficult.
To the other point, LeBron indeed was within his rights to exercise his free agency, but the right to change teams does not absolve him from any criticism. Every choice carries with it risks. He made a choice knowing the risks that he would alienate certain fans, especially those in his home state.