The government is basically saying they can't fire him for being an alcoholic because hey, that's mean.
Not mean. It's discrimination. (Edit: well, negative discrimination is mean, but "it's mean" is not the reason why the government is saying they can't fire him.)
Entirely ignored is the incontrovertible fact that even "former" alcoholics are much more likely to relapse than a random person is to become an alcoholic in the first place, and thus the driver poses an unnecessary safety risk.
Again, you're implying that being an alcoholic means you're posing an unnecessary safety risk, and that non-alcoholics don't pose comparable risks. If you read the topic, you'll see that we've already had this debate. I'll repeat the most important points for your convenience:
1) A non-alcoholic could also get drunk and get behind the wheel. That's also an "unnecessary safety risk."
2) The driver in question was apparently working for five years without incident, so even if he was posing a risk, it didn't exactly show in his output at work. Which is all that should matter. Like Igloo said, what the truck driver does off the clock is none of the employer's business.
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Full rap metal jacket ~ Method Man