You are forgetting the word "purely." If some fascist incites people to riot and innocent police officers get killed (hypothetical example), then there are strong political reasons to put the perpetrators behind bars for their crimes. Law and order is a political issue. Protection of institutions is a political issue.

The First Amendment doesn't say you can incite a riot as long as it is a political riot. That a crime is politically motivated (or politically impactful) does not somehow absolve the perpetrator.