I'm in college, and I work in childcare... EVERY party I go to here is at least partially unhosted, ranging from all food provided, just BYOB, to potluck, to meeting up in restaraunts and everyone paying their own bill. BUT, the kind of party it is, is always indicated. I know whether I need to bring a dish, a drink, or money. Also, even if it all is in one cooler, all booze belongs to the person who brought it, though sharing is NOT uncommon.
As for the story, I bet it wasn't a BYOB party, thus the rudeness. I also wonder if the hosts drink alcohol? My best friend's mom doesn't allow alcohol in her house, and she has had at least one guest who REPEATEDLY brought alcohol to her house (her husband's coworker), though that was several years ago now.