I am not a lawyer, and this question would probably be more suitable for Law.SE as your question is more of a legal issue, but I will tell you what I know.
The principle behind "At-Will" employment is that either you or your employer can terminate the relationship at any time, for any reason, with or without cause, and with or without notice. In practice, however, the world operates very differently.
In the specific case of alcohol consumption that is on your own time, in the absence of a policy forbidding you from doing so, would be a legally murky case for firing you. It all depends on the circumstances and the type of work you do.
First off, here are the easy things that a company can't fire you for:
- Federal law prohibits discrimination against the usual issues of race, gender, religion, national origin, age, sexual identity, disability, genetic information, or military service. Also, there are federal statutes that extend protections to other activities, such as whistleblowing, or a leave of absence due to the the Family Medical Leave Act (FMLA) for example.
- State laws offer many protections above and beyond federal laws. These vary from state to state. You have to check with yours specifically, but many of those protections govern things like moral/ethical objections to certain job duties, various lifestyle choices, etc. Some states even have statutes regarding alcohol and tobacco use specifically.
- Companies cannot fire you for your refusal to violate the law, work overtime, work "off the clock", or endure threats or harassment that would be considered illegal for them to do.
- If you're a government employee, have an employment contract, work for some companies that do federal contracts, or are a member of a labor union, you have some more protections there. Government employees in particular cannot be fired for exercising their Constitutional rights, which a private company could fire you for. This is because the Constitution limits how the government may act, but does not limit how private citizens may act.
Now, here are things a company definitely can without question fire you for:
- Criminal activities, whether or not those activities were connected with your employment. This generally excludes civil offenses, such as judgements in private lawsuits, traffic vilations, etc -- unless your job depends on these things.
- Any violation of written company policies, including your own job description. Obviously, if your job description is to do X, you can be fired for not doing X. But most companies also have ethics policies (which can be broad in scope), dress codes, computer use policies, non-disclosure policies, and yes, even tobacco and alcohol use policies that you can be fired for. You generally have to agree to these before you begin working for the company, and usually you sign a document to that effect (though not always). Companies have a lot of freedom to implement whatever policies they choose (so long as they don't violate the legal protections above).
- Anything which brings public shame or embarrassment upon the company. Employees can and have been fired over negative comments about their employer on social media for example. You could also be fired for calling into question the company's legitimacy or expertise in their industry. For example, a financial counselor can be fired if they mishandle their own finances, a social worker could be fired for not paying child support, a delivery driver for having unpaid traffic tickets, etc. Lack of professionalism would fall into this category as well.
- Being unfit for the job. If you are or become physically or mentally unable to perform your job duties then you can be fired. This could qualify as a disability, but the law says they only have to make reasonable accommodations for you that don't impose an undue burden on them. The terms "undue burden" and "reasonable accommodation" have legal definitions.
I'm sure there are things I haven't thought of that could be placed on both lists, but most everything else I haven't mentioned here is a legal gray area. Technically, the law says they can fire you without cause, but practically speaking, you also have a right to sue your employer and make up whatever argument you want. Due to that legal uncertainty, this means in practice, companies usually build an internal case against you so they can fire you with cause. If at-will employment were to be interpreted as loosely as some of the other answers here suggest, companies would rarely bother with write-ups, employment evaluations, and documented disciplinary processes. A with-cause dismissal is very difficult for you to defend, and lawyers are expensive.
There's also the issue of unemployment compensation. A without-cause dismissal usually entitles you to an unemployment check which the company has to pay for (the specifics of which also vary from state to state). A with-cause dismissal usually doesn't.