Going on some political tirade isn't any reason to suspect he would have a weapon. Even if for some reason they did think he had a weapon, it would have become clear he didn't when he was pinned on the ground for a few minutes by a couple cops. It would have been slightly different if they had used to taser to actually restrain him, though still unjustified, but at the point they did taser him he was restrained and clearly no real danger to anyone. They had restrained him but were trying to put the cuffs on, and he wouldn't budge so they tased... The other option was getting beaten with one of their sticks cause he sure as hell wasn't moving.