The amount of street lighting is a complex function (varying a great deal in space and time) of local budgets, changes to CFL/ led/ other technologies, and political desires on the one hand for the perception of fighting crime, and on the one hand much more rarely for less light pollution. I do not see that they controlled for any of this (and can't imagine how they could have). Possibly some of the cited papers that I did not look at might have tried to address this.