Actually, it is what employers are using when they are choosing to drug test and reject applicants - federal law. The universities are using that also. So, while, CO has legalized using marijuana, employers are able to still “discriminate” and “drug test” because of overriding federal law.
Preventing the discrimination suits, so far. They are bound to come, though.
And yes, an employer can have performance expectations. I have not heard of employers not hiring/firing someone for tobacco use in CO, but I know they are pushing things like not having smoking areas on work property, offering smoking cessation programs free of charge.
They don't need any reason whatsoever if the state is an at-will employment state. Unless, perhaps, they cite federal law because they are bound by union contracts (and "violations of federal law" or some such language provides them with some CYA material).
Preventing the discrimination suits, so far. They are bound to come, though.
There will be no (successful) anti-discrimination lawsuits. If so, they would have already happened with the employers that test for tobacco and/or alcohol.