Was there actually? In many cases--I don't know about this one--what has increased is the reporting of violence, even while the actual violence has gone down.

I don't think, for instance, that the police response to student protests at People's Park in Berkeley in 1969 (shooting buckshot at the crowd, injuring 50) or at Kent State in Ohio in 1970 (live ammunition, 9 injured, 4 killed) is nearly as likely now as it was back then. This is, admittedly, the official response, but the cultural prohibitions against violence seem like they were lower back then.