If you really want to know the law, you can look at:

https://help.cbp.gov/app/answers/detail/...or-personal-use

http://www.ttb.gov/importers/personal_importation.shtml

Different states have different rules. Here is Virginia's

Returning from vacation outside of Virginia?
The Code of Virginia prohibits the transportation or importation of more than one gallon of alcohol into the Commonwealth per individual per occasion. You do not need any paperwork to bring back one gallon or less.

Many states have a one gallon rule.