League of Legends is a privately owned establishment.

And like any privately owned establishment, the owners have every right to refuse your entrancd if they disagree with your behavior. Your freedom of speech, if you wish to cite that, does not hold precedent over the wishes of your hosts. Your freedom of speech does not hold you unaccountable from the consequences that the owners of this place wish to give you for saying or doing the things you do. A permaban is a forced removal by the owners for breaking their policy.

Furthermore, because metaphors are loved on this board, a permaban is not "getting arrested for your words." You aren't arrested. You aren't held in custody, unable to go anywhere else like DOTA or Smite or something. You aren't given a permanent record of toxicity that other other can look at and deny you your privilege to their establishment preemptively. A permanent ban is the same as going into a restaurant and breaking their no cell phone policy, or going into a gaming cafe and harassing other customers, or going into any other privately owned establishment and breaking rules. Treat this place like you would any other place you've been allowed entrance to.

tl;dr: follow Riots rules and you'll keep your privilege to use their services.