I submit a radical idea to you: it's not really whether parties serve alcohol or not, it's who shows up wishing to attend and what expectations/ideas of "fun" are possessed by the hosts and available attendees. I could name you several s-f conventions where alcohol is served at parties - and it plays a minor role as a social lubricant for people who really want to talk to each other. The ID issue is handled in a low-key way.

What you wind up with is partly a function of the way the entire convention is marketed and programmed -- which affects the nature of the attending population and the night time party environment.