My experience has been that beer in consuites is very much a regional thing. Most Southern US cons tend to have it freely available. And of course the whole idea of drinking in room parties promoting bids, etc, is that if you get the punters lubricated enough, they are more likely to part with their money. ID checking at the door vs at point of service is going to vary from one local authority to another. I don't think that you can rationalize 18 to 21 year olds not being allowed access to the entire convention, for the purpose of paying less. The parties are not put on by the convention itself, but rather by the specific groups that they are promoting. They can attend all of the convention events.

One solution used commonly in the South is to have different coloured badges for the under drinking age adults. That means that ID checking gets done at registration. Of course, that doesn't prevent people from swapping badges later on.

I suspect that more fans drink than you actually realise.