Because arrest does not mean convict, and his actions consisted of more than speaking out. He has publicly admitted to something that, from the surface facts, appears to be illegal (regardless of whether you think it was morally right and acceptable). As such, until convicted or exonerated, it is the job of the justice system to bring him in for trial.

It's comparable to killing someone in what you believe was self defense. You may still have to go to trial to prove your innocence if the government believes it may not have been self-defense. That is the point of the justice system, and trial by a jury of your peers.

The right to bear arms is to protect from overreach by the government. As long as government consists of representatives of the people making laws in accordance with the constitution and prior laws, there should not be a case where arms are required to defend against the state. It ensures the government cannot monopolize force.