It's a perception thing, and that is more important the higher the level you go. At supply level and above, and certainly at contrib, you aren't that likely to see the players drinking alcohol after the game as that is not something that any sportsperson should do (after exercise you should never touch alcohol until you have drunk enough water to turn your urine clear, which I am sure we all know …
). There's an expectation at these levels that the referees have minimum fitness levels, and tucking into a pint after the game just doesn't look very professional as you should have worked so hard you need to re-hydrate. You also need to remember that you as an assistant aren't being marked by the clubs, the referee is, so you have to take his lead on things.
It isn't just alcohol. I remember turning up to one game as an assistant when the other assistant turned up with a McDonald's coffee and the referee wasn't happy. It drew comments from players, and whilst he had only stopped there for a coffee it didn't portray the right impression to turn up to a semi-pro game with something from a fast food restaurant .