I don't think it's that. I think it's more of a misconception of what's protected and what isn't.

If someone is prescribed a medication that causes them to create an unsafe environment (falling asleep, not using equipment properly, injuring someone) because that medication impairs them, they can be fired.

If the medication does not impair their ability to do their job, they are protected.

If someone is currently using illegal drugs, they can be fired.

If they have previous history of using illegal drugs, they are protected.

If a person is drunk at work, they can be fired.

If they get drunk at home and it interferes with their ability to do their job the next day, they can be fired.

If they get drunk at home and can still do their job, they are protected.

It's a common misconception that drug addicts cannot be fired for only being a drug addict. No one is entitled to keep their employment simply because of an addiction. If a person is no longer able to do their job with reasonable accommodation because they suffer addiction, or because they developed a brain tumor, or because a mental illness has become unmanageable, etc. No matter the cause, the fact remains they are unable to do their job.

Some companies off short and long term disability to their employees which can help if they are not able to continue working. However, a company is not and should not be required to keep an employee who is unable to do their job.