I always thought the immunity was a voluntary one, akin to a Jedi not needing air for 24 hours by slowing his heart rate, stopping respiration and concentrating his meditative powers into quelling the need within him. For monks who gain such immunities, such is the case I would think, and for druids it is possible that they use their shifting abilities to "partially shift" certain internal organs, blood types, etc to purge the poison at will. Since alcoholic beverages are best to be enjoyed while drunk, they would naturally lower their resistances to the toxins meant specifically for inebriative effect. Fighter classes and dwarves do not get the option unfortunately, as their resistances are built up via their systems building tolerance to the effects. They start drinking ale as young lads and by the time they're old a grouchy, it takes a good swat of hard liquor to rosey up those cheeks. If druids or monks wanted to enjoy good tasting beverages without getting drunk, berry juice is far less fattening and healthy to consume.
I believe the principle of voluntary immunity also explains why not a single drow has innately resisted a healing spell, buff or pickmeup, despite their body's "natural" resistance to magic. We're only worried about stopping that which harms us, not deadening all the boons of life.
