From a legal standpoint, Dash punching that guy out is a big risk, but seeing what happened to that guy will hopefully make other fans less likely to jump the steel railing. I know that back in the day, wrestlers attacking fans that tried to get involved was more about keeping kayfabe and showing that wrestlers were stronger than random fans, but I imagine it's also always partially been about giving other fans an example of what bodily harm will come to them if they dare to do something stupid.

Granted, mentally unstable people are never going to think rationally.