Many substance-abuse disorders are not treated as special-accommodation-worthy (in courts often or by HR departments by policy) in the the US. Alcoholism is one such example (Alcohol Abuse Disorder is not a qualifier for disability status without additional presenting symptoms, or exceptional luck during disability determination).
That reality, and the reality of addiction, presents a few problems for the hypothetical employee here:
- Attending work events that do have alcohol may cause them to endure a great deal of hardship (talk to a recently-abstinent alcoholic if you doubt this).
- Addiction has no conclusive test or diagnosis--AAD and other indicators are often not present in people who enter rehab, or in people whose substance abuse is identified as a primary motivator for criminal behavior by courts. This means that "getting trashed and acting like an ass", for "real addicts" (whatever that means, which is a troublesome qualifier to add in and of itself) is difficult to prove to be the fault of the company providing alcohol, and for non-addicts is a convenient out (if provided to the former group).
- "Actually required to go to an event" is another troublesome category. Many events aren't "required" . . . unless you want to get promoted/not eventually get fired in favor of someone who attended. I don't propose some legal solution to this (everything I can think of would effectively be thought-policing), but it's an important ambiguity to acknowledge.
- Even if a humane HR/management department exists to whom the hypothetical employee could disclose their condition as a disability, and even if that department lobbied the employee's managers/colleagues to prevent addiction from being a disadvantage to their career, that would still likely result in either a breach of that employee's privacy or eventual prejudice seeping in (e.g. via turnover inside HR) regardless. Not good.
- If those recourses fail, and the employee ends up before the courts pleading wrongful-termination or equivalent based on their addiction, the (at least state) US court system and arbitration organizations are notoriously inconsistent and prejudiced against claims of addiction as any sort of mitigating or complicating circumstance. A company interested in preserving the autonomy, promotability, and dignity of addicted employees would likely view the courts as something the employee in question should be kept away from for their own benefit.
There are many other considerations.
Now, many of those apply to any uncommon disability condition, and it could be argued that below a certain point a very few employees' accommodations should not ruin the fun for everyone else. Even if you buy that argument, the incidence of addiction/substance-abuse related serious lifestyle trouble—principally at work or in romantic relationships—for very large numbers of people in the US is well documented.
Perhaps it would be better to simply forbid the creation of such situations on the company dime.