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.
I am telling you, I have not heard of a company in CO testing for tobacco, just illicit drugs.