I'm not sure why the cops tested the victims of the crash as they were not driving but were just the passengers. If they had focused only on the person responsible and not the victims this whole issue would have been avoided.
It's for anti-alcohol stats purposes, i.e., propaganda. If the guilty driver had been sober this would still have been alcohol-related crash for stats purposes as the not-responsible-for-the-crash dead had been drinking.
Because of the pressure to classify every "crash" as "alcohol-related", thereby creating a demand for more and strict laws.
By the way, they used to be called "accidents", which connotes an air of unavoidability.
Now they're called "crashes" which creates an opportunity for legislation against a perceived cause.