The law prohibits bartenders from serving drunk patrons or from serving alcohol to a person reasonably expected to become drunk (or drunker) on the drink. 
Personally, I blame both if the customer was served when acting drunk (it's like pornography, we all know it when we see it). As far as decisions go: wouldn't you prefer the bartender to err on the side of safety when he knows the guy is driving? 
In a perfect world, alcohol wouldn't be an addictive drug and drinkers would all stop drinking before inebriation. They don't. Reasonably, then, many states have determined that holding the profit-makers liable is the only way to cut down this dangerous practice. Bars are in the business of selling as much booze as they can. If left alone (obviously, not all of them), they'll send off any number of customers to drive who barely can walk out the door. 
Both the drinker and the bartender/tavern owner should be responsible: alcohol is dangerous stuff.