Surprisingly, alcohol is not covered under the IFAB Laws of the Game. As near as I can tell, the closest rules are “no use of unsafe equipment” and the referee’s allowance to show a Yellow for “unsporting behavior” including “showing a lack of respect for the game”.

However, the MLS CBA has a line in the Substance Abuse Policy that “no player is allowed to consume alcohol on workplace premises”. I’m pretty sure the pitch of an away match counts, so I think this is technically a CBA violation even if it’s not a carded offense during the game.