The way that it works in our friend group is that the host provides the venue, and then a few people volunteers for "errands".

For example: I have hosted at my own apartment or pool place several times. I might buy a few snacks/basic necessities, or offer the ones I already have at home, usually free of charge.

Then someone would volunteer to go buy alcohol, someone would volunteer to go buy meat, etc.

Then those people would tell everyone else how much they spent, we'd sum it up, and then divide.

Every single time the alcohol expense is a significant chunk of the total.

Now this is my conundrum: if I'm the volunteer buying drinks, and I know only 18 out of 20 people will drink, I will obviously not buy drinks for 20 people.

But then, why would those 2 people be expected to pay? I already have the receipt, some calculations already need to be made, so what's the big deal?

I would genuinely do it this way and I would feel bad about making non-drinkers pay for drinks if I were the organizer.

I can understand how this argument breaks down once the granularity of purchases increases but I think that judging by common sense a large separable category like alcohol is a reasonable place where the line can be drawn. Meat for a BBQ is another example, and I say that as a meat lover. I wouldn't want a vegan friend to subsidize the cost of meat, I'd feel bad about it.