That's correct. Employers can enforce rules that are important to them for any reason, unless they're violating Constitutional rights - such as "we don't hire women" or "we don't hire ... [name your minority group]."

And if there is a safety or performance reason, employers will never lose that argument. For example, it has been perfectly legal since prohibition ended to drink to excess - with exceptions, like if you are driving. But an employer can fire you on the spot if you are drunk at work ... and especially if you are around machinery where someone could get hurt. It won't be any different with legalized drugs. Only the detecting/testing will be different.