You're confusing a statistical claim for an absolute claim.

You're right that if the claim is "drugs are never, ever the cause of any violence" then you just need one counter-example.

But the claim is actually "statistically, violence caused by drugs does not occur at a rate detectably above the rate of violence in general" which means something like that the number of violent acts with drugs as a cause is extremely low, or that violent acts caused by drugs would still have happened just with a different cause if it were not for the drugs.