I don't think that is the complaint. Referee bias is just human nature. People have a hesitancy to make a decision that will immediately cause tens of thousan

I don't think that is the complaint. Referee bias is just human nature. People have a hesitancy to make a decision that will immediately cause tens of thousands of people to scream at them. That likely is the primary explanation here. Yet there is nothing done to isolate the refs outside of just averaging play among leagues and assuming that takes care of differences in play style. However players are impacted by fans just like referees. There can be a reasonable hypothesis that visiting players play less aggressively in empty stadiums which could also explain some of the observed changes. The numbers included in the article are not enough to draw conclusions on what is causing these changes.