That's an asinine standard. Banning drunkenness at work has a disparate impact on alcoholics in the office.
It's a two step process. In that case, the rule has a disparate impact on people who are protected by certain laws (the ADA), but the basis behind the rule isn't to discriminate against people who are protected by the ADA, it's meant to increase on-the-job safety as well as productivity.
I agree, though, that the rules have become somewhat byzantine. My wife represents employers in employment law issues, so I hear some great stories.