I would think it can be argued that flipping a bird” constituted “fighting words” and is NOT protected by the First Amendment. That being said, striking someone for doing so would also likely NOT be seen as a justifiable act. On the other hand, the totality of the protester’s actions might be portrayed as threatening and violent to such a degree that a “reasonable man” feared for his safety. Unlikely, but possible.
Hardly possible to represent a person IN CUSTODY as being threatening. Of course, it is possible that his lawyer might try to use that as justification, but still.
That first sentence contradicts itself.