I am pretty positive employers already have that Right.
Of course they do because the drugs are currently illegal. If an employer wants to terminate employment for someone on prescription meds it gets more dicey and they have to document how it is affecting their performance. If street drugs were legalized it would have to be with the caveat that employers can have a zero use policy and conduct random and targeted testing.
Alcohol is legal, and employers have the right to terminate someone who shows up drinking or that they suspect of being drunk. Most employment is 'at will' to begin with and I am sure if drugs were legalized it would not change the status quo, employers can already test employees for drugs, employers can fire employees for legal (alcohol, pain meds, etc) drug use, I don't think much would have to be done.