Why wouldn't they continue with the programming, and be ready to press charges against anyone who behaves violently?
That doesn't really follow from the facts as a reason to be concerned about violence. Austin's crime rate is lower than that of most other major American cities, including ones such as Chicago which have heavy gun control. (As per Wikipedia, Chicago's murder rate for 2014 was 15.1, Austin's 3.5.)
Though I can see them being overly cautious as a result, I think context is important here: that was a drunk driver running from the cops. SXSW being there was a tragic coincidence.